Gottfried Leibniz's theory of pre-established harmony is a philosophical theory about causation under which every "substance" only affects itself, but all the substances (both bodies and minds) in the world nevertheless seem to causally interact with each other because they have been programmed by God in advance to "harmonize" with each other. Leibniz's term for these substances was "windowless monads".
- An apple falls on Alice's head, apparently causing the experience of pain in her mind. In fact, the apple does not cause the pain - the pain is caused by some previous state of Alice's mind. If Alice then seems to shake her hand in anger, it is not actually her mind that causes this, but some previous state of her hand.
Under pre-established harmony, the preprogramming of each mind or body must be extremely complex, since only it itself causes its own thoughts or movements, for as long as it exists. In order to appear to interact, each substance's "program" must contain a description of either the entire universe, or of how the object is to behave at all times, during all "interactions" which will appear to occur.
Therefore, the use of Occam's Razor is one possible response to the concept of pre-established harmony: it is simpler for there to actually be a universe of interacting objects than a universe of non-interacting objects that must each behave as if they were interacting with an entire external universe.
It can also be noted that if a mind behaves as a windowless monad, there is no need for any other object to exist in order to create that mind's sense perceptions, leading to a solipsistic universe consisting only of that mind.