Gottfried Wilhelm Leibniz, answered from the texts and cited to the page.
There are three ways, and only three, to explain the agreement of soul and body and of substances with one another.1 The way of physical influence, that one substance acts upon another by transferring something, is unintelligible. The way of assistance, Malebranche's occasionalism, has God intervene at every moment to make the one answer the other — a perpetual miracle.
There remains pre-established harmony: God so constituted each substance at creation that, unfolding its own law from within, it agrees with all the others as if they influenced it. The monad has no windows; nothing enters or leaves it; each draws its whole succession of perceptions from its own depths and yet expresses the entire universe from its point of view. The harmony alone honors both the self-sufficiency of substance and the manifest correspondence of soul and body.234
However it is reasonable to attribute true motion to bodies in 76 B.THE RELATIONSHIP BETWEEN SOUL AND BODY accordance with the supposition which explains phenomena in the most intelligible way, this denomination being in accordance with the notion of action that we have just established. 3.The Shorter Leibniz Texts, p. 77
The result, by a kind of miracle, was that even when their strokes had been intentionally disturbed, they came to beat together again, somewhat like two strings tuned to each other. The second way of making two clocks, even poor ones, agree always is to assign a skilled craftsman to them who adjusts them and constantly sets them in agreement.I. A New System of the Nature and the Communication of Substances, as well as the Union between the Soul and the Body, 1695, p. 472
But I hold that this is to invoke a Deus ex machina in a natural and ordinary thing, where according to reason God should only intervene in the way that he concurs with all other natural things.The Shorter Leibniz Texts, pp. 78–79
It is this that I call the theory of pre-established harmony, which excludes every concept of miracle from purely natural actions and makes things run their course regulated in an intelligible manner.Considerations on Vital Principles and Plastic Natures, by the Author of the System of Pre-Established Harmony, 1705, pp. 599–600
The doctrine for which Leibniz was best known to his contemporaries, presented in the 1695 New System. The standing objection is that the denial of all real interaction is deeply counter-intuitive and purchases its explanatory tidiness at a high price; the contemporary debate concerns whether harmony is genuinely superior to occasionalism or a notational variant of it, and how windowless monads can be said to express a world they do not touch.