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3. Occasionalism and pre-established harmony
Descartes’s philosophical influence was such that he came to have a considerable number of philosophical followers (see, for instance, Arnauld, A.; Clauberg, J.; Cordemoy, G. de; Desgabets, R.; Fardella, M.; Henricus Regius; La Forge, L. de; Le Grand, A.; Régis, P.-S.; Rohault, J.). Among these Cartesians, many were also occasionalists. The most famous of the early modern European occasionalists was Nicolas Malebranche, author of the The Search after Truth (1674–5) and Dialogues on Metaphysics and on Religion (1678).
Occasionalism is, in the first instance, a view about causation. In its simplest and strongest version, it is the view that only God has causal powers (see also Causation, in modern philosophy). All other things – all finite, created things – lack causal powers. Thus even if finite beings, including humans and their minds, seem to be the causes of things, there is an important and fundamental sense in which they are not, because only God can really cause something to happen. Things other than God are said, rather, to be only occasional causes. Thus, for example, on the occasion of my deciding to make some tea, I walk to the kitchen and put the kettle on. But neither I, nor my decision, is really the thing with causal power. Only God has that. So it seems we must say that, according to the occasionalist, it was God, not me, who caused the change in the state of the kettle (see also Occasionalism).
Occasionalism is, then, a general view about causation and about God’s relationship to the world. However, it can also appear to provide an answer to the puzzle about mind–body interaction that Elisabeth posed for Descartes. Put simply, the answer is that the mind can have no effect at all on the body. But this is not because of some defect of the mind, or some weakness of Descartes’s system. Rather it is because the mind is a finite, created thing, and no created things have any causal power at all, for that belongs only to God. The mind does change on the occasion of changes in the body, but this is the work of God, not of the motions of corporeal things.
One of occasionalism’s most prominent opponents was Gottfried Wilhelm Leibniz. He attacked the occasionalist view for postulating too many miracles. To think that God, rather than some finite substance, was the cause of some change in a finite substance, was to think of God as intervening miraculously into the world every time something happened. Leibniz took this to be the wrong sort of explanation:
in solving problems it is not sufficient to make use of the general cause and to invoke what is called a Deus ex machina. For when one does that without giving any other explanation derived from the order of secondary causes, it is, properly speaking, having recourse to miracle.
(Leibniz  1989: 143)
Leibniz’s own system also contained surprising claims about causation, however. In his picture, finite substances still do have some causal powers: an earlier state of a substance is genuinely the cause of a later state of it. But Leibniz thought that no finite created substance could, fundamentally, cause a change in any other. Rather, he believed that all the created substances evolved harmoniously. Suppose, for example, that you say something (‘Leibniz wore a wig’) and a friend listens to you and comes to believe it. Their believing it does have a cause, but fundamentally for Leibniz that cause was an earlier state of your friend, not your saying what you did. God, however, created you and your friend so that your states would be synchronized, the belief arising just after your utterance. Leibniz calls this pre-established harmony.
Like occasionalism, this is a general view about causation, which might be employed in answer to puzzles about mind–body interaction. As Leibniz says, ‘It is this mutual relation, regulated in advance in each substance of the universe, which produces what we call their communication, and which alone brings about the union of soul and body’ (Leibniz  1989: 144). Here, as indeed with occasionalism, there are complications. Not the least of these involves understanding what Leibniz thinks bodies actually are: many have thought that he was an idealist (see §5). But we can at least see a possible view here: states of a mind are explained by its earlier states, states of a body are explained by its earlier states and neither mind nor body exercises causal power on the other, but their states evolve harmoniously, thanks to the way that God created the world.
Duncan, Stewart. Occasionalism and pre-established harmony. Mind and body in early modern philosophy, 2016, doi:10.4324/9780415249126-V043-1. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/mind-and-body-in-early-modern-philosophy/v-1/sections/occasionalism-and-pre-established-harmony.
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