Locke, Leibniz, and the Logic of Mechanism

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early mechanist philosophers demanded a new standard of perspicuity in the natural sciences. They accused others of “explaining” phenomena in terms of obscurely defined, unconfirmed, and uninformative causes. These complaints were leveled, not just at the real qualities and forms of Scholastics, but also against the sympathetic attractions of Hermetics and the sophic principles (mercury, sulphur, salt) of the Spagyrites. These competitors to mechanism could at best demonstrate that a certain effect occurs and claim to state why it does. Mechanists aspired to explain how phenomena are produced.¹ Beyond that, however, philosophers in the broadly “mechanist” movement shared no single account of what this sort of explanation involves. They were too diverse, not only in their views on the central notions of material substance, cause, and force, but also with respect to their standards of intelligibility. In this paper, however, I want to isolate one mechanist model of explanation with intellectual virtues that strongly recommended it in some quarters. I will argue specifically that it influenced Leibniz and Locke.

This favored form of mechanism opens a deep problem, because it conflicts with the existence of causal interaction between bodies and minds.² It is not just that the model does not apply beyond the realm of intercorporeal phenomena, but much worse, that acceptance of mental-corporeal causality threatens to undermine the mechanist model in its most perspicuous form. The difficulty could be avoided, but by accepting a form of mechanism considerably less intellectually satisfying. The difference has to do with the metaphysical analysis of what it is for an individual to have a causal power. Hence Leibniz and Locke and perhaps others faced a dilemma: whether to affirm the more appealing version of mechanism or to affirm a mechanism that accommodates mind-body interaction. The former required one account of causal powers, the latter a significantly different one.

Leibniz had reasons for affirming the more perspicuous version of mechanism and the associated account of causal powers; and he denied mind-body interaction. I intend to show that the specifically mechanist model of intelligibility plays a little noticed part in his arguments against psychophysical causality. But in this paper I will have more to say about Locke. He was convinced that we observe mind-body interaction and he gave experience the highest authority. My contention is that plain logic required him to reject the favored version of mechanism. He noted that bodies have some powers that are inconceivable to us (e.g., as he stressed, powers to cause ideas). These are powers of the sort Locke repeatedly said we can only ascribe to the good pleasure of God. There has been a protracted debate over the correct interpretation of these remarks.³ I will argue that they express Locke’s logically cogent response to the dilemma. Recognizing that mechanism cannot explain some powers bodies have, as he thought experience shows, Locke proposed that a version of mechanism might still be correct; but in that case, some corporeal powers have no intelligible means of manifestation. They operate only by “the good pleasure of God” and their conjunction with intelligible corporeal powers has the same source. But he was not so committed to any version of mechanism that he affirmed that there are unintelligible corporeal powers.

There has been some interest in tracing the early modern antecedents of later views that identify causal relations with regularities or with general laws that contingently govern things of the sorts they do.⁴ An account of the first type was later urged by Hume, whereas the second was advocated by the occasionalist, Malebranche. Neither is intelligible by the mechanist standard, which locates causality in individuals whose powers are the basis of general laws. Leibniz was able to maintain that all causes in nature are of this sort. Locke, who could not see his way clear to that, still refrained from adopting an alternative view of causality. But the difficulty psychophysical causality poses might seem to make a less realist theory attractive.

2. the favored version of mechanism

The contributions to theoretical natural science made by Leibniz and Robert...