PHILOSOPHY PATHWAYS Issue 187

The articles in this issue discuss the work of three thinkers who tried, in rather different ways, to accommodate their religious beliefs within a philosophical world view -- or conversely, in the case of our first paper, to employ philosophical argument in attempting to deal with difficulties arising for an avowedly theological world view. Though their careers were separated by a relatively short space of time -- the late sixteenth century to the early eighteenth -- that period includes the beginning of the Enlightenment, and the position of God in the work of these three men is very different.

Kevin Kimble discusses the work of Spanish sixteenth-century theologian Luis de Molina, who developed the notion of 'scientia media' -- 'middle knowledge' -- as a way of solving two closely related problems: how God can have infallible knowledge of causally indeterminate future events; and how divine foreknowledge, which is fixed and infallible, can be reconciled with the contingency of such future events. Kimble concludes that, ingenious though it is, the idea of middle knowledge, intermediate between God's 'natural knowledge' of essential truths and his 'free knowledge' of contingent facts about the world raises problems of its own, and thus does not offer a distinctive contribution to solving the problems it was devised to address.

Rene Descartes famously developed the idea of substance dualism ('Cartesian dualism'), the view that there are two fundamental kinds of substance, mental and material -- thus making room in his philosophical world view for the immortal soul insisted upon by Christianity. Jani Kukkola's paper considers the implications of the intentionality, or 'aboutness', characteristic of mental states, which is sometimes regarded as the strongest argument in favour of substance dualism. He concludes that it supports, instead, an epistemological dualism: different kinds of knowledge, rather than different kinds of substance.

Sim-Hui Tee's paper examines Leibniz's account of the relation between force and motion. Leibniz's philosophy of physics incorporates his notion of 'primitive active force', which underlies other forces and must first be created by God before any motion can take place in the mechanical universe. The paper discusses a fundamental problem for Leibniz's account: how can primitive active force, which in his metaphysical scheme corresponds to immaterial 'substantial form', lead to the production or alteration of motion in material bodies? Leibniz offers two solutions to this difficulty, both of which the paper finds to be inadequate.

Do the points made in these three papers entitle us to draw any more general conclusions about the relationship between religious belief and philosophy? Probably not: they deal with very specific issues, and we should be cautious about extrapolating too far beyond them. Nevertheless, they do serve to illustrate some of the problems and complications that can arise when theses that incorporate, or seek to defend, premises derived from religious faith are exposed to the rigour of philosophical argument.

The Medieval period witnessed a rigorous debate between various thinkers over the problem of the compatibility of divine foreknowledge, on the one hand, and contingent truths about the future (future contingents), on the other. This problem is a two-sided coin that can be stated more clearly by posing two distinct but closely intertwined questions: (1) how can God infallibly know causally indeterminate future events? and (2) how is divine foreknowledge, which is fixed and infallible, to be reconciled with the contingency of such future events? The Spanish Jesuit theologian Luis de Molina (1535-1600) espoused an intriguing and very controversial theory, called the doctrine of scientia media or middle knowledge, in an attempt to come to grips with these and other questions. In Part IV of the Concordia Molina lays out in detail the major tenets of his doctrine of middle knowledge and the way in which the theory allegedly solves the traditional riddle concerning divine foreknowledge and the contingency of future human actions.[1] After sketching Molina's theory of the Scientia Media, I go on to assess his claim that the theory satisfactorily answers the two questions about divine foreknowledge.[2] I shall argue that Molina fails to give a plausible solution to (1), and that his answer to (2), even if it succeeds, is not a unique contribution of the doctrine of middle knowledge.

The fullest explication of Molina's positive account of divine knowledge is found in Disputation 52 of the Concordia. Central to his theory is the curious doctrine of 'middle knowledge'. Molina's main motivation for espousing the doctrine is that it is required in order to explain the manner in which God acts providentially in the world and predestines his creatures for certain ends. Without middle knowledge of contingent events, God could not know how to order the means to achieve His ends, and thus could not predestine certain ends through (free) secondary causes. Molina describes three consecutive 'moments' in God's knowledge of the actual world. The first of these he calls 'natural knowledge'. Such knowledge is logically or conceptually prior to God's decision to create, and comprises all metaphysically necessary truths along with all possible states of affairs and complexes of states of affairs that could hypothetically obtain. In Molina's own words,

     One type is purely natural knowledge, and accordingly could
     not have been any different in God. Through this type of
     knowledge He knew all the things to which the divine power
     extended either immediately or by the mediation of
     secondary causes, including not only the natures of
     individuals and the necessary states of affairs composed of
     them but also the contingent states of affairs -- through
     this knowledge He knew, to be sure, not that the latter
     were or were not going to obtain determinately, but rather
     that they were indifferently able to obtain and able not to
     obtain, a feature that belongs to them necessarily and thus
     also falls under God's natural knowledge.[3]

For on the hypothesis and under the condition that God should will to create this or that order of things, the divine ideas represent to God naturally, before any free determination of His will, every future contingent state of affairs under that hypothesis and condition.[4]

According to Molina, then, God's natural knowledge has two crucial features: (a) it is prevolitional, in the sense that God possesses this knowledge (logically) prior to His decision and will to create, and (b) the content of this knowledge is essential to God, in that God not only comprehends all metaphysically necessary truths essentially, but He also knows all possibilities essentially, including what each indeterministic secondary cause is able to do in any possible situation in which it is in a position to act.

Molina goes on to describe a second type of knowledge, which he dubs God's free knowledge. God's free knowledge is His knowledge of this existent, actual world. It differs from natural knowledge in two crucial respects. First, such knowledge is conceptually posterior to His will and free decision to create, and hence is post-volitional. In addition, its particular content is not possessed essentially by the divine nature, but rather is contingent upon which world (out of all the possible orders) God in fact decides to create. But these two divisions of knowledge are not enough together to ground God's knowledge of absolute future contingents. God's natural knowledge tells Him only what each free secondary cause is able to do, but not in fact what it would do, in each possible situation. And although His free knowledge informs Him of the total causal contribution He Himself makes to the world He decides to create, the fact that His general concurrence with secondary causes is intrinsically neutral prevents His causal contribution to the contingent effects of secondary causes from uniquely determining what those effects will be. Thus Molina needs to introduce yet a third division into the structure of divine knowledge:

     The third type is middle knowledge, by which, in virtue of
     the most profound and inscrutable comprehension of each
     faculty of free choice, He saw in His own essence what each
     such faculty would do with its innate freedom were it to be
     placed in this or that or, indeed, in infinitely many
     orders of things -- even though it would really be able, if
     it so willed, to do the opposite... [5]

Middle knowledge combines the prevolitional aspect of God's natural knowledge with the contingent feature of His free knowledge. Middle knowledge is that aspect of divine knowledge, prior to any determination of the divine will, which encompasses the particular decisions and actions finite wills would freely make under any hypothetical set of circumstances, along with the total set of their effects. To illustrate, let's take one of Molina's favorite examples, Peter's denial of Christ. With respect to God's complete knowledge of Peter, God knows prevolitionally by His natural knowledge everything that is metaphysically possible for Peter to do, that is, every action that Peter could perform or might perform, in any possible set of circumstances in which Peter might be placed. God knows, for instance, any other possible decisions or courses of action that Peter might have taken on the night that he betrayed Christ. He also knows all the different choices Peter might have made and the ways his life might have turned out had he been placed in various different sets of circumstances (e.g., if he had not been a fisherman and had never met Jesus). Moreover, God knows prevolitionally according to His middle knowledge, out of the vast sea of possibilities, what Peter would in fact do (or would have done) if placed in any particular set of circumstances. Perhaps if Peter had been a merchant instead of a fisherman, he would have (freely) moved his family to Rome. Then God knows by His middle knowledge that if those circumstances had obtained, then Peter would have moved his family to Rome. Finally, God knows post-volitionally, following His decision to create the world that is actual, what Peter will do in the circumstances which actually obtain. So God always knew (post creation decision) that Peter will in fact deny Christ.

According to Molina, middle knowledge cannot be reduced to either natural knowledge or free knowledge, but shares features of each. On the one hand, middle knowledge is not part of God's free knowledge because (i) it is logically prior to any free act of the divine will, and (ii) the content of God's middle knowledge lies outside the scope of His control. Divine knowledge of the particular contingent events that would occur under any set of circumstances is independent of God's actual decision to create a given set of circumstances, and is in fact a prerequisite for God's knowledge of the future unfolding of events. And since middle knowledge is (logically) prior to the divine will, His knowledge of each conditional future contingent that comprises the particular content of His middle knowledge is simply given and not within the scope of His control.[6]

On the other hand, neither can God's middle knowledge be assimilated to His natural knowledge. For whereas the specific content of God's natural knowledge consists of necessary truths and hence is essential to Him, the content of His middle knowledge is not, for it is metaphysically possible for free creatures to act differently in the exact same set of circumstances. Middle knowledge is similar to natural knowledge in that the content of each is not dependent on any divine decree to create, while it is similar to free knowledge in that the content of each is contingent and dependent on free acts of will (in the former case decisions of free creatures, in the latter case the free activity of God).

3. Molina's application of the scientia media to the problem of divine foreknowledge

Having briefly explored Molina's doctrine of middle knowledge, I now consider Molina's attempt to utilize the theory in resolving the problem of divine foreknowledge. Recall how the traditional problem of divine foreknowledge can be formulated by asking two questions: (1) how can God know certainly and infallibly the occurrence of causally indeterminate future events? and (2) how is such fixed and infallible foreknowledge to be reconciled with the contingency of future (free) actions and events? The answer Molina gives to the first of these questions involves two distinct stages. The first stage is Molina's unequivocal claim that it is God's middle knowledge coupled with knowledge of His will and decision to create that form the basis of His foreknowledge of future contingent events in the actual world. There is first God's pre-volitional middle knowledge of conditional truths about possible futures, by which He knows which contingent effects would emanate from any arrangement of circumstances and secondary causes. In addition to that, God has post-volitional knowledge of the total causal contribution He wills to make to the created world, including which secondary causes He will create and the particular arrangement of circumstances in which He will place them. This provides God with complete free knowledge of all absolute future contingents. By choosing to actualize a certain order of hypothetical states of affairs, God knows what further states and events will be actual as a consequence.

Molina also appeals to this first stage of his answer to (1) in trying to meet the often noted objection that external events cause God's knowledge, and hence that knowledge is causally dependent on something outside of God. Molina insists that God does not acquire His knowledge from external sources, but knows all future contingents in Himself. God's knowledge that a certain state of affairs will obtain is grounded in His decreeing that the state of affairs obtain. And this divine decree is guided by God's pre-volitional middle knowledge of how certain possible secondary causes would act in various possible sets of circumstances. In other words, God's (post-volitional) foreknowledge of any future event or state of affairs is grounded in His creative decree to order the world in such a way that the states of affairs in question will be produced with God's concurrence through the relevant secondary causes. Hence there is no appropriate sense in which it can be claimed that entities and events external to and independent of the divine being and will are the bona fide cause of His foreknowledge.

One might well wonder whether invoking middle knowledge to explain simple foreknowledge simply pushes the problem one step back to the question of how God possesses middle knowledge of conditional future contingents. Along similar lines, one might wonder (in response to the above objection) whether it is the case that God's middle knowledge is caused by states of affairs external to and independent of Him. This is precisely where the second stage of Molina's answer to (1) comes in. In answering the question of how God has pre-volitional knowledge of the contingent effects secondary causes would produce given the obtaining of various hypotheses or circumstances, Molina invokes the notion of divine comprehension or super-comprehension. Because of God's 'infinite and unlimited perfection, by which He comprehends each created faculty of choice in a certain absolutely profound and eminent way',[7] God's intellect infinitely surpasses the capabilities of finite free wills so that He understands them so thoroughly that He knows not only what they could choose to do under any set of circumstances, but also what they would choose. Molina elaborates:

     Before (in our way of conceiving it, but with a basis in
     reality) He creates anything at all, He comprehends in
     Himself -- because of the depth of His knowledge -- all the
     things which, as a result of all the secondary causes
     possible by virtue of His omnipotence, would contingently
     or simply freely come to be on the hypothesis that He
     should will to establish these or those orders of things
     with these or those circumstances; and by the very fact
     that through His free will He established in being that
     order of things and causes which he in fact established, He
     comprehended in His very self and in that decree of His all
     the things that were in fact freely or contingently going
     to be or not going to be as a result of secondary causes.[8]

In explaining the basis of God's middle knowledge of conditional future contingents, Molina appeals ultimately to God's cognitive perfection. Middle knowledge demands a kind of cognitive power ascribable to the supreme deity alone, a super-comprehension by which the deity must surpass in perfection by an infinite distance the creature known, and must have epistemic certitude regarding states of affairs and events that do not (before they occur) have metaphysical certitude. Thus Molina's answer to the question of how God knows absolute future contingents involves a two part solution. His explanation appeals to a free determination of the divine will to create free creatures in certain circumstances along with the divine comprehension in His essence of each created free will through His middle knowledge.

Does Molina provide a satisfying answer to question (1)? If the doctrine of the scientia media is to shed any real light on the problem as part of a helpful explanation of how God has divine foreknowledge of certain kinds of events, then the doctrine must do more than merely explain God's knowledge of one kind of contingent truth in terms of another that is just as mysterious. But this is precisely what Molina himself appears to do.[9] Moreover, if the source and basis of divine foreknowledge of simple future contingents is thought to be (as it was for many medieval thinkers) the common notion of God's profound and preeminent comprehension of His creatures and their actions, and if such a notion has yet to be explained, then it is difficult to see how Molina's own appeal to a kind of super-comprehension of conditional future contingents amounts to any real progress in satisfactorily explaining God's foreknowledge. For the notion of super-comprehension is itself rather mysterious and unexplained, and Molina's discussion leaves it at a somewhat vague and abstract level.

Familiar 'Thomistic' solutions to (1) tend to ground divine knowledge of future contingents strictly in the determination of the divine will whereby God decrees and determines even the effects of secondary causes; and in this way divine foreknowledge can be seen as a species of causal knowledge, albeit of a special sort (about the future). But defenders of divine foreknowledge who, like Molina, affirm an indeterministic or 'libertarian' notion of free will and contingency cannot employ this same strategy. There are simply no analogous metaphysical underpinnings available to explain how it is that God certainly and infallibly knows, for example, that Peter would deny Christ if placed in circumstances C, when in fact it is fully consistent with the obtaining of those exact same circumstances that Peter not deny Christ in C. To that extent Molina's answer to (1) seems severely impoverished in comparison with other suggestions offered from within the Thomistic camp, and his answer fares no better than certain other solutions which are sympathetic to libertarian conceptions of free will. I conclude, then, that with regard to question (1), Molina's appeal to middle knowledge does not improve upon other explanations found within the Medieval tradition.

What about the second half of the foreknowledge problem, as expressed in our question (2)? How is the theory of middle knowledge supposed to reconcile the tension between the certainty and infallibility of God's foreknowledge and the contingency of the actions and effects of secondary causes? I think the best way to understand Molina's position here is by turning to the argument he discusses for theological fatalism:

1) (Assumption) Suppose that it is true at time t that p, i.e. that God foreknew 80 years ago that S will do A.

2) Closure of necessity under logical entailment (CL): If p is absolutely necessary, and p entails q, then q is absolutely necessary.

3) Fixity of the past (FP): For any proposition p about the past that is true at t, no agent (including God) has the power at or after t to bring it about that p is false.

4) Absolute necessity (AN): Any proposition that is 'fixed by the past' according to (FP) is absolutely necessary.

5) Conclusion: therefore, q is absolutely necessary (S will do A is absolutely necessary).

Molina clearly accepts both FP and AN, contrary to the Ockhamist line of response. He holds that were some future contingent, foreknown by God, not to obtain, then God's infallible middle knowledge would have been different, and God would thus not have foreknown that the event would occur under those circumstances. Molina is careful to distinguish the following two kinds of claims:

(A) God's knowledge that p is now such that, having known that p, God is able not to have known that p.

(B) God's knowledge that p is such that He might have never known that p at all.

Molina vehemently denies (A) but gives assent to (B).[10] In other words, it does not follow from the contingency of future states of affairs that God's knowledge of them is able to be different than it in fact is; it follows only that were the future contingents not to occur, then God's knowledge of them would have been different all along, since He would have known from eternity other future contingents instead. To illustrate with one of Molina's favorite examples, there is an absolute necessity now involved in God's knowing that Peter would sin. When God in actual fact possesses that piece of knowledge at t, it cannot then or at a later time be negated or erased in any way. But it is possible that God had never known that Peter would sin under the circumstances, just in case Peter would not have sinned in those circumstances. And in that case God's middle knowledge -- His knowledge of certain conditional future contingent states of affairs -- would have been different.[11]

We are now in a better position to see how Molina attempts to circumvent the argument for theological fatalism, namely by rejecting the only remaining principle in the argument -- the closure principle CL. And that is precisely what Molina does:

     Even if (i) the conditional is necessary (because in the
     composed sense these two things cannot both obtain, namely,
     that God foreknows something to be future and that the thing
     does not turn out that way), and even if (ii) the antecedent
     is necessary in the sense in question (because it is
     past-tense and because no shadow of alteration can befall
     God), nonetheless the consequent can be purely
     contingent... if the truth of the antecedent is posited (as
     is in fact the case), then the consequent is necessary only
     with a necessity of the consequence, by which it is validly
     inferred from that antecedent, and not with a necessity of
     the consequent, since the condition in question does not
     render the consequent absolutely necessary in the way that
     it does render the antecedent absolutely necessary. The
     consequent is not affected by it in any way, but is instead
     unqualifiedly able to obtain and able not to obtain.[12]

Molina claims that the antecedent of a conditional may be absolutely necessary but the consequent contingent, in cases in which the necessity involved is temporal necessity and the knowledge claim in the antecedent depends on the contingent fact that the state of affairs known was going to obtain.[13] The power of secondary causes to act freely is not closed under entailment. Hence it is within Peter's power not to sin, but it is not within his power to alter God's foreknowledge; even if Peter were to refrain from sinning under the given circumstances, then God would always have known the complement of that particular conditional future contingent, and that proposition would have been absolutely necessary instead. So Molina's strategy is to attack CL, and if CL is false, then the entire argument for theological fatalism is rendered untenable.

Is it a plausible move to deny the closure principle? One consideration weighing in favor of the plausibility of rejecting CL is that many philosophers have regarded as problematic other parallel closure principles which play a role in philosophical argumentation. For example, there is the famous epistemic closure principle which has been attacked by Fred Dretske and others. In discussions of ethical reasoning and the logic of ought, forms of the deontic closure principle have come under fire. Finally, in the free will debate, closure principles involving what actions are within an agent's power to do (sometimes referred to as power necessity principles) have also been sharply criticized as untenable.[14] All this is just to say that there is some precedent from these other cases for casting prima facie doubt on FP and for thinking that Molina's strategy might be a viable one.

An alternative solution open to a Molinist to pursue goes as follows. Molina claims the consequent of a conditional of the form 'If God foreknows that p, then p' is contingent, such that the agent is able to perform or refrain from performing the action described by p. Moreover, we have seen that Molina clearly distinguishes assertions of the form of (A) from those of type (B). Thus it seems that he ought to endorse the following claims:

(iii) S has it within his power to refrain from performing A at t*, and if S were to do so, then God at t would have foreknown the negation of p.

In other words, Molina can accept that agents have a kind of counterfactual power over the past and claim that counterfactual power is all that is required for genuine, libertarian free will. Given this, Molina can then concede the entire line of reasoning in support of fatalism, including the conclusion of the argument, without undermining the contingency of future free effects of secondary causes. The sense in which p is said to be absolutely necessary in the argument is too weak to undermine free will and genuine contingency, for no one thinks that such free will requires agents to have causal -- or to put it more pointedly, retro-causal -- power over the past. Thus it appears that Molina can escape the unwelcome consequences implied in the fatalistic argument even if he concedes the truth of CL.

However, both of the above Molinist responses have a significant shortcoming. Neither solution constitutes a unique explanatory contribution of the doctrine of middle knowledge per se, for neither solution need invoke the doctrine in order to preserve the consistency between divine foreknowledge and indeterministic free will. Molina's preferred solution involves denying CL in cases of temporal necessity, but the rationale he gives appeals to God's free knowledge, not his middle knowledge. The alternative reply suggested circumvents the fatalistic argument by invoking an agent's counterfactual power over the past and claiming that such power suffices for the ability to do otherwise, but once again, the reasoning is not tied to any doctrine of middle knowledge.

Molina's theory of middle knowledge is an ingenious attempt to come to grips with the traditional problem of divine foreknowledge. He offers what I take to be a tenable strategy for answering question (2), but such a strategy leaves no room per se for middle knowledge to make any special contribution. With regard to (1), I have tried to show that in order for the doctrine to be helpful in explaining the basis of God's foreknowledge of future contingents, Molina owes us a more satisfying account of what constitutes the ground of divine middle knowledge itself.

Craig, William Lane. The Problem of Divine Foreknowledge and Future Contingents from Aristotle to Suarez. Leiden: E.J. Brill, 1988.

Flint, Thomas P. Divine Providence: The Molinist Account. Ithaca, NY: Cornell University Press, 1998.

de Molina, Luis. On Divine Foreknowledge (Part IV of the Concordia). Translated by Alfred J. Freddoso. Ithaca, NY: Cornell University Press, 1988.

Normore, Calvin. 'Future Contingents', in Norman Kretzmann, Anthony Kenny, and Jan Pinborg, eds., The Cambridge History of Later Medieval Philosophy. Cambridge: Cambridge University Press, 1982, pp. 358-381.

1. Luis de Molina, On Divine Foreknowledge (Part IV of the Concordia). Translated by Alfred J. Freddoso. Ithaca, NY: Cornell University Press, 1988. Unless otherwise specified, all references to Molina that follow in this paper are to this translation.

2. In order to restrict the scope of this paper to these points, I will not weigh in on issues regarding the fruitfulness of applying Molina's views to other doctrines such as divine providence, prophecy, and petitionary prayer. Nor do I have the space to discuss one important objection to Molina's doctrine that has been ably articulated and defended by two contemporary anti-Molinists, Robert Adams and William Hasker. For a thorough review of Adams and Hasker's objection , see Flint.

6. As is the case, presumably, with mathematical truths, logical truths, and any other necessary truths that form part of His natural knowledge.

9. A similar point is made by Calvin Normore in his paper 'Future Contingents,' p.380.

14. See, for example, Ted Warfield and Keith DeRose, eds., Skepticism: A Contemporary Reader (NY: Oxford University Press, 1999), pp. 1-26; H. E. Mason, ed., Moral Dilemmas and Moral Theory (Oxford: Oxford University Press, 1996), pp. 31-58; and John Martin Fischer, The Metaphysics of Free Will (Cambridge, MA: Blackwell, 1994) pp. 23-45.

In this short article I will review Cartesian dualism and intentionality. The latter has been considered by many as the most convincing argument for Cartesian substance dualism. I will study the relationship between dualism and intentionality, and evaluate whether intentionality really supports such a view.

Rene Descartes' substance dualism argues that there are two fundamentally different kinds of things: physical and mental things. According to his method of doubt, Descartes claims that everything physical can be doubted; they can be treated as if they were false. In the case of the human mind, the doubting ends. Because I can doubt and I can be fooled (by an evil demon) to believe false things, the mind itself that doubts cannot be doubted. Thus, the physical body can be doubted, but the mind cannot. The mental stuff of mind is fundamentally different from the stuff of the body or the brain. Mind is a thinking and body an extended thing. The body extends itself so that the intellect can perceive things in the world.

Descartes argues that the mind and the body/ brain do not share all the same properties. The brain is divisible into parts: brain cells and functional areas. The mind on the other hand is a unity, which is not divisible into smaller parts. As Descartes puts it: 'I am unable to distinguish any parts within myself.'[1] Another property the mind and the brain don't share is the capability for introspection. One can think of one's own mind, but one cannot come to know about one's brain by merely thinking about it. There is also difference in spatiality: the body/ brain is spatially located, but the mind isn't as it is impossible to point out where thoughts exactly occupy space.

Intentionality is often referred to as the 'aboutness' of mental states. As Franz Brentano and later phenomenologist Edmund Husserl put it, intentionality is directedness towards the world and its objects. To have intentional mental states is to be directed towards something in the world: the thought that the painting I see is beautiful is about the painting, but this 'aboutness' is not reducible to the painting nor to my nervous system. Thus, intentionality, or 'aboutness', is not part of the physics of the world or my brain chemistry. Intentionality has been considered as perhaps the strongest argument favoring dualism.[2] Intentionality poses a problem for physicalist positions: intentional states are essentially holistic, but there appears to be no counterpart of this feature in purely physical states of affairs. The mind is qualitatively different from non-mental, purely mechanical things, because of the fact that mind is intentional. It seems, that intentionality is not physically constituted, as no physical thing possesses the same properties as intentionality does. However, it does not follow from this, that intentionality by necessity favors substance dualism.

Intentionality is an emergent property; it is not reducible to other, more primary physical features of a biological organism. It is impossible to explain how such reduction occurs, at least on the basis of our current scientific knowledge. But even though we cannot explain how such reduction occurs, it does not mean that it doesn't happen: intentionality could emerge from those physical properties. Intentionality is not, thus, necessarily an argument for (ontological) substance dualism, but it functions as a knowledge-argument, favoring epistemological dualism. We cannot know how intentionality emerges from physical properties, but even though we lack the knowledge, we ought not necessarily to conclude from this that substance dualism is correct. Even though intentionality and the immediate experience we have of it seems to fit well with Descartes' thinking of the introspection and indubitability of our own minds, this experience does not necessarily require two different kinds of stuff, mental stuff and physical stuff, but only two different kinds of knowledge of things. Those two kinds are (1) the subjective, irreducible, intentional experience of things in the world, and (2) the objective knowledge of things in the world. From the perspective (2) we cannot get to know what it is like to have knowledge (1). As Thomas Nagel[3] explains, we can observe bats and get to know their behavior or have understanding of their sonar sense, but we can never have the first person perspective knowledge of what it is like to be a bat. Intentionality is a property of the knowledge (1), it is a necessary part of the first person perspective. In knowledge (2) we can observe things in the world and we can even expect them to be true to others, regardless of them having any personal experiences of those things. There is thus an unbridgeable gap between the two types of knowledge, and this is what I think is in the heart of the problems with Cartesian dualism and intentionality. Therefore, the problem with mind and matter is not necessarily a problem of two different substances, but of two different kinds of knowledge.

As I have shown, the relationship between intentionality and Cartesian dualism is not as symbiotic as many would argue. Intentionality is a strong but not a definitive argument for substance dualism. In fact, the relationship between intentionality and dualism appears to be more problematic than it may first seem. Intentionality does not only support substance dualism, but one could also argue in favor of another kind of dualism from it. This other dualism is epistemological dualism, which points out a certain perspectiveness behind the constitution of different types of knowledge.

1. Descartes, Rene. 'Meditations on First Philosophy,' in Chalmers, David. (ed.) Philosophy of Mind. Classical and Contemporary Readings (New York: Oxford University Press, 2002), p. 18.

2. Brentano, F. 1995. The Distinction between Mental and Physical Phenomena. In: D. Terrell, A. Rancurello & L. McAlister (eds.) Psychology from an Empirical Standpoint. New York: Routledge; Horgan, T. & Tienson, J. 2002. The Intentionality of Phenomenology and the Phenomenology of Intentionality. In: D. Chalmers (ed.) Philosophy of Mind. Classical and Contemporary Readings. New York: Oxford University Press, pp. 520-533; Chalmers, D. 1996. The Conscious Mind. New York: Oxford University Press, p. 19; Robinson, D. 2008. Consciousness and Mental Life. New York: Columbia University Press, pp. 87-88; Frank, M. 2012. Ansichten der Subjectivitat. Berlin: Suhrkamp, pp. 223-224; Foster, J. 1982. The Case for Idealism. London: Routledge & Kegan Paul.

3. Nagel, Thomas. 'What Is it Like to Be a Bat?' (Philosophical Review, 1974), pp. 435-50.

Leibniz argued that motion is one of the properties of extension which constitutes physical bodies. This interpretation of motion is clearly geometrical and mechanical, which is characteristic of his contemporaries. However, Leibniz's mechanical worldview incorporates a new element which is not mechanical in essence, that is, the induction of substantial forms. Substantial forms are needed in metaphysics, according to Leibniz, for they are the necessary basis for active force, which makes a substance genuine. However, Leibniz denies that substantial forms have mechanical effects on the physical phenomena, though the derived active force is substantial in explaining the mechanical bodies.

     ... the whole nature of bodies is not exhausted in their
     extension, that is to say, in their size, figure and
     motion, but that we must recognize something which
     corresponds to soul, something which is commonly called
     substantial form... (Leibniz 1908, 18)

The properties of extension, which is an intrinsic property of all physical bodies (including living ones), presuppose the properties of inertia and impenetrability. These latter inactive properties constitute the passive force, which is the resistant force of extension. Besides, motion presupposes the existence of active force in physical bodies, without which the physical bodies would remain in the state of rest. Active force, which is grounded in substantial form, is the mirror of God's action. Active force that is imposed on a physical body is a necessary characteristic by which motion can be distinguished from rest. Leibniz holds that force (both active and passive) is the property that makes possible motion of physical bodies.

Leibniz views the relation between force and motion in two ways. First, he maintains that force is realized through motion, without which the force cannot come into existence. In a letter to Burcher de Volder dated 1699, Leibniz maintained that active force 'exercises itself through motion.' (Leibniz 2006, 128). The nature of this active force is governed by the perpetual law. According to Leibniz, the primitive active force must be first created by God before any motion can take place in the mechanical universe. According to Duchesneau (2008), primitive force can be equated with fundamental laws governing the changing nature of physical bodies at the phenomenal level. However, this interpretation does not take into account the primitive force as the God's initial creation that gives rise to the derivative forces. Because Duchesneau's equating primitive force with fundamental laws is taken to be effective at the phenomenal level, it fails to explain the divine properties that Leibniz was originally attributing to the primitive forces.

Apart from characterizing the force as a property realizable by motion, Leibniz has taken the relation between force and motion as a causal relationship. He states that force is the cause of the motion by which the latter arises from it. However, this mechanical interpretation of the relation between force and motion is not without problems. First, as force is a substantial form, it cannot possess mechanical properties. In other words, the mechanical effects exhibited by motion are force-independent and must not lie in force, for the mechanical movement cannot be realized without a physical body yet it is conceivable in the absence of force. The notion of force as a non-mechanical substantial form renders the causal account untenable. Second, the problem persists even if we grant that the mechanical property of motion, which lies in the physical bodies, is induced by force. There remains a question of how an immaterial force can cause (induce) a mechanical motion. It is unlikely for Leibniz to have recourse to the first cause, for the force in the universe is not directly imposed by God upon physical bodies. This difficulty is exhibited by various names conferred to 'force' over years in Leibniz's writing: 'motive force', 'moving force' (Loemker 1969, 297), 'living force' (Loemker 1969, 438), 'absolute force' (Loemker 1969, 639), 'force of elasticity' (Leibniz 2006, 126), and 'final causes' (Leibniz 1908).

Despite the difficulty faced by Leibniz in accounting for the relation between force and motion, these properties lay the foundation for the phenomena of matter, and should be accounted for through recourse to metaphysical rather than physical consideration if one aims to understand the principles. However, Leibniz does not deny that the notion of force has its place in the laws of nature.

     This consideration of the force, distinguished from the
     quantity of motion, is of importance, not only in physics
     and mechanics for finding the real laws of nature and the
     principles of motion, and even for correcting many
     practical errors which have crept into the writings of
     certain able mathematicians, but also in metaphysics it is
     of importance for the better understanding of principles.
     (Leibniz 1908, 32)

Notice that in the quoted paragraph above, Leibniz has restated the problem of 'the distinction between force and motion' as the problem of 'the distinction between force and quantity of motion'. To understand the nature of force in physics and metaphysics, Leibniz holds that one should view force as a property which is different from the quantity of motion rather than from the motion. This is evidenced in Machina animalis, where Leibniz has disconnected motion from force by stating that the motion of animal bodies may be described merely by 'animal economy' (the study of the relation between organs and their functions) without taking force into consideration (Smith 2011). Motion is taken to be a functional property rather than a physical property. Quantity of motion, in contrast, is the physical property of the moving object. It is defined as the velocity multiplied by the mass of the moving object. Leibniz further elaborates this point by claiming that motion is unreal (non-physical).

     Because motion, if we regard only its exact and formal
     meaning, that is, change of place, is not something
     entirely real, and when several bodies change their places
     reciprocally, it is not possible to determine by
     considering the bodies alone to which among them movement
     or repose is to be attributed... (Leibniz 1908, 32)

However, Leibniz has recognized that a fixed quantity of motion is inadequate in explaining the activities of physical bodies, especially the living ones. Therefore, the notion of motion serves as a principle that accounts for the functional aspect of physical bodies. Taking quantity of motion and motion together, the movement of a physical object can be explained in terms of physical property and functional property, respectively, without being able to explain the cause of the movement. Still, a convincing account of the relation between force and motion (or quantity of motion) is required.

Leibniz attempts to provide such an account by conferring on both force and quantity of motion a mechanical status of realness. By so doing Leibniz implies that force and quantity of motion are real physically, in the sense that the change of place is brought about in space.

     But the force, or the proximate cause of these changes [of
     place] is something more real, and there are sufficient
     grounds for attributing it to one body rather than to
     another, and it is only through this latter investigation
     that we can determine to which one the movement must
     appertain. (Leibniz 1908, 32)

Granting that force is mechanically real, it seems to solve the problem of the causal relationship between force and motion. However, this Pyrrhic victory has incurred an enormous difficulty in defending Leibniz's position of the substantial forms. This is because the mechanical nature of force cannot be explained in the non-mechanical interpretation of substantial forms, for force is derived from the latter. Besides, difficulty arises in the distinction between motion and the quantity of motion. Because Leibniz has characterized motion as unreal (non-physical) while the quantity of motion is real (physical), it remains elusive how the latter can be derived from the former.

In conclusion, Leibniz's account of the relation between force and motion is unsatisfactory. Leibniz has attempted to solve this problem using two strategies. In the first strategy, force is taken to be either: (1) an immaterial (which is consistent with substantial forms) property which could be realized by motion; or (2) the cause of motion. This account of the relation between force and motion fails to explain how the immaterial force can (1) be realized by physical motion; or (2) cause the movement of physical bodies. Leibniz's second strategy in restating the relation between force and motion as the relation between force and quantity of motion fares no better. By conferring both force and quantity of motion a mechanical status of realness, Leibnizian account of substantial form and force is incoherent.

Duchesneau, F (2008). 'Rule of Continuity and Infinitesimals in Leibniz's Physics,' In Infinitesimal Differences: Controversies between Leibniz and his Contemporaries. Ursula Goldenbaum and Douglas Jesseph (ed). Berlin: Walter de Gruyter. pp. 235-253.

Leibniz, G (1908). Discourse on Metaphysics, Correspondence with Arnauld and Monadology. George Montgomery (Trans). Chicago: The Open Court Publishing Company.

Leibniz, G (2006). The Shorter Leibniz Texts: A Collection of New Translations. Lloyd Strickland (ed). London: Continuum.

Loemker, L.L. (trans. and ed.) (1969) Leibniz: Philosophical Papers and Letters, Dordrecht-Holland: Reidel.

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