Occasionalism is often thought of primarily as a rather desperate solution to the problem of mind-body interaction. Mind and body, it maintains, do not in fact causally affect each other at all; rather, it is God who causes bodily movements to occur 'on the occasion of' appropriate mental states (for example, volitions), and who causes mental states, such as sensations, on the occasion of the corresponding bodily states (for example, sensory stimulation).
This characterization, while correct so far as it goes, is seriously incomplete. Occasionalists have seen the lack of real causal influence between mind and body as merely a special case of the more general truth that no two created beings ever causally affect each other. The one and only 'true cause' is God, with created beings serving as the occasions for his causal and creative activity, but never as causes in their own right. (The one possible exception to this is that created agents may themselves bring about their own acts of will; this is necessary if they are to be in any sense free agents.) Occasionalism has always been held primarily for religious reasons, in order to give God the honour due to him as the Lord and ruler of the universe. It has never, however, been a majority view among philosophical theists.
The first thinker clearly to articulate an occasionalist position was the Muslim theologian al-Ghazali. He wrote in defence of orthodox Islam against the philosophers al-Farabi (§2) and Ibn Sina (§5), both of whom propounded emanationist systems based on a combination of Aristotelianism and Neoplatonism (see Creation and conservation, religious doctrine of §3; Aristotelianism in Islamic philosophy §2; Neoplatonism in Islamic philosophy §§2-3). One of al-Ghazali's fundamental objections to the emanationist scheme was that it was necessitarian and denied God the freedom due to him as creator and as the author of miracles. (For al-Ghazali, as for other orthodox Sunnis, human freedom was not a major concern.) Over against this, he affirmed the teachings of orthodox Ash'arite theology, according to which all natural beings are completely inert and the true and sole agent in nature is God (see Ash'ariyya and Mu'tazila). Thus, there is no necessity in nature itself which could constrain or limit God's omnipotent will. Unlike the Ash'arites, however, al-Ghazali presents a philosophical argument for this position. The only form of necessity he recognizes is logical necessity, and he has little difficulty in showing that causes do not logically necessitate their effects. The relation between what we take to be causes and effects is merely one of correlation and is purely contingent; the one real, productive cause of all things is God. In addition to this philosophical argument based on a critique of causality, al-Ghazali appeals directly to the theological dogma of God's absolute omnipotence. Indeed, al-Ghazali's vision of God is such that God is virtually the only true existent: 'There is no other being with Him, for Him to be greater than it.... [N]one has being save through His Face - so that His Face alone is' (quoted in Fakhry 1958: 72).
Al-Ghazali's rejection of philosophy was disputed by Ibn Rushd, but his views became widely accepted throughout Sunni Islam. His occasionalism was sharply criticized and rejected by both Maimonides (§4) and Aquinas, and never enjoyed wide support during the Christian Middle Ages. It was, however, embraced by the late medieval Ockhamist philosophers Pierre d'Ailly and Gabriel Biel.
Occasionalism in modern philosophy took its 'occasion' from the problems over causality that arose in Descartes' philosophy (see Descartes, R. §§8, 11, 13). (Not only the problem of mind-body interaction, however; causal interaction among bodies was also problematic for Descartes.) Descartes himself had tendencies towards occasionalism, and this view was adopted by the Cartesian philosophers Louis de La Forge and Géraud de Cordemoy. Arnold Geulincx may have been the first to state an important principle that is also found, in modified form, as a premise in arguments concerning causation given by Malebranche and Hume. The principle states that something cannot be done unless there is knowledge of how it is done; more specifically, people do not do what they do not know how to do. This quickly disposes of the contention that the mind causes bodily movements; in order for the mind to do this, we should have to know how to control the physiological processes involved in making such movements - the firing of neurons, for instance. Clearly, however, we do not know this. The principle also excludes causal efficacy for corporeal bodies, since these bodies obviously lack knowledge of how their supposed effects are produced.
Nicolas Malebranche (§4) was clearly the most significant of the modern occasionalists. Basing his argument on the supposed impossibility of mind-body interaction, he writes that 'There is no real relation between one body and another, between one mind and another. In a word, no created thing can act upon another by an activity which is its own' (1688: 4.11). He argues, like Geulincx, that in order for our minds to be able to move our bodies we should have to understand, in full anatomical detail, how this is done. He also argues that God's causality pre-empts, as it were, any possible causality among creatures. No creature can exist unless God, by 'continuous creation', wills that it should exist. But in willing that a chair (for example) should exist, God must will that it exist in some particular place, in some particular state of motion or rest, and so on. Clearly, no created power (assuming there to be such) can cause the chair to be at rest or in motion unless God wills that it be so. But if God does will the chair to move, then necessarily it moves; there is nothing left to be done by any created agent. Thus, 'God communicates His power to created beings only because He has made their modifications the occasional causes... which determine the activity of His volitions in consequence of the general laws which He has prescribed to Himself' (1688: 7.10).
Occasionalism did not persist for long as a widely accepted view. But the influence of Malebranche was considerable, and is still felt today through Hume, many of whose sceptical arguments concerning causation derive from Malebranche.
Is occasionalism simply a historical curiosity, or does it have the potential to become a viable position for present-day theists? Leaving aside Geulincx's far-from-evident principle, there are two principal arguments for occasionalism, based respectively on the critique of natural causality and on the pre-emptive causality of God. The argument from the critique of natural causality proceeds as we have already seen in al-Ghazali and Malebranche. Various criticisms are employed (many familiar to us through Hume) to argue that causality as we experience it in nature is not the necessary, genuinely productive relationship that we ordinarily take it to be. Natural causation having thus been disposed of, God is invoked as the 'true cause' for whose efficacious acts the various states of created beings are mere occasions. This argument, however, is dialectically weak. The occasionalist begins (as we all must) from our ordinary intuitions about causality developed through our ordinary, everyday causal interactions with the world. These intuitions are then undermined through the critique of causality. But then the occasionalist invokes those very same intuitions in order to point to God as the truly efficacious and productive cause of all that occurs; causation by God, however, is not subjected to the kind of critical analysis that has been applied to natural causation. Clearly, this procedure is inconsistent. If the critique of causality is as effective as occasionalists think, then (as Hume rightly saw) divine causation is, if anything, worse off than natural causation in consequence.
In view of this, the main burden of support for occasionalism needs to be borne by the argument for the pre-emptive causality of God, also employed by both al-Ghazali and Malebranche. (The argument of Malebranche based on continuous creation is a special case of this.) Classical theists are in agreement that God's causal activity must be seen as universal and pervasive throughout the created world. To suppose that there is some part of created reality in which God's activity is not involved is just to make that part of the creation independent of God, which detracts from divine dignity. Any attempt to circumvent this problem by designating different aspects of causation which pertain respectively to God and to creatures runs up against the complaint that the aspect assigned to creatures is removed from the sphere of God's activity, thus diminishing the honour due to God. Medieval Aristotelians tried to meet this challenge by the theory of dual causation, which claims to give full value both to the divine 'first cause' and to creaturely 'second causes'. This theory was spelled out in their doctrines of divine conservation and divine concurrence with creaturely action, but no detailed consensus was arrived at (see Creation and conservation, religious doctrine of §5). Contemporary philosophical theology has barely begun to address the issue, so it could fairly be urged that on this point occasionalism presents theistic philosophy with a challenge it has not yet met.
Whether this argument provides effective support for occasionalism depends on whether occasionalism is itself a tenable view. It can be argued on both epistemological and metaphysical grounds that the material substance posited by occasionalists is redundant and should be eliminated, leading to a Berkeleyan immaterialism (see Berkeley, G. §§3, 6-7). Epistemologically, occasionalism holds that material substances make no causal contribution towards our perceptions of objects. Occasionalists hold, to be sure, that light waves (for example) travel from observed bodies to our eyes, that nerve impulses connect the eyes with the brain, and that consequent to this we experience visual images. But there is no real causal connection between any two of these stages; in particular, the visual images are not produced by the brain (or even by the immaterial mind), but rather by God 'on the occasion' of the brain's being in an appropriate state. This means, however, that we have no direct evidence for the existence of the supposed intermediate stages in the process, and it is hard to avoid Berkeley's conclusion that it would be simpler for God to omit them.
A similar conclusion can be reached through metaphysical considerations. Occasionalism holds that created substances make no active causal contribution to subsequent states of the world. But then the occasionalist is faced with the question of whether created substances are endowed with causal powers at all. If they are, then God must somehow intervene to prevent the powers from being exercised - and indeed, the provision to created things of powers that are never exercised and are not intended by God to be exercised would seem highly unreasonable. So the occasionalist must deny that created beings possess causal powers. But what, if anything, might created substances consist in, if they lack such powers entirely?
Apparently the occasionalist answer is that while created substances lack active powers they do possess some passive powers; hence al-Ghazali's view of material beings as inert, and Malebranche's willingness to attribute movability and impenetrability (but not active power) to material objects. So as Freddoso suggests, 'God would supply all the active causal power in nature, while material substances would receive and channel God's causal influence as patients' (1988: 111). The difficulty with this is that the active/passive distinction, as we make it, cannot give occasionalism what it needs here. We do have some grip on the difference between an object's acting on another or being acted upon by another. (In many cases this would coincide with the direction of energy transfer.) But the very same molecular structure which makes a billiard ball impenetrable by another billiard ball also enables it, under appropriate conditions, to propel the other billiard ball in a desired direction. The active/passive distinction just does not cut deep enough to play the role it is cast in by occasionalism. But if material substances have neither active nor passive powers, then it is difficult to see the point of their existing - indeed, it is difficult to see what could be meant by claiming that they exist.
The correct conclusion would seem to be that the logic of occasionalism leads to Berkeleyan immaterialism, and that it is in this form, if any, that it may be able to survive as a viable contemporary option. A more attractive option, though slightly more remote from classical occasionalism, can perhaps be found in certain anti-realist interpretations of natural science (see Scientific realism and antirealism). Though diverging from the letter of both occasionalism and Berkeleyanism, these anti-realist views are akin to them in spirit in their desire to uphold science as an empirical enterprise while avoiding the mechanistic materialism which threatens on a realistic interpretation of scientific theories.
See also: Causality and necessity in Islamic thought; Edwards, J. §3; Miracles; Religion and scienceWILLIAM HASKER