For Leibniz, substances always act. In fact, they act forever, they act forever, even though they are finite, because they are naturally indestructible. But what is the source of their causative power? Is God the only causal agent in nature, like the occasionalists taught? Does God merely create and conserve substances, which then have causal power of their own? These are the kinds of questions concerning causation that the 17th century philosophers were wrestling with.
Leibniz, for his part, chose a doctrine of pre-established harmony instead of the competing physical influx and occasionalist choices. The physical influx view, as taught, for example, by Francisco Suarez, was that there is a kind of inflow between cause and effect. This means that there is “intersubstantial” causation among finite substances. Thus, when Felipe pets a chow chow, there is real, causal interaction between Felipe’s hand and the chow chow.
Motion is the mode of his hand and it is transferred or communicated to the head of the chow chow. Although this theory is known as the “physical” influx theory, it does not apply solely to finite substances. It can also apply to causal interaction between immaterial finite substances. This is important for Leibniz, because he believed that monads were substances and that these substances were immaterial. The physical influx theory was held by Thomas Hobbes, Pierre Gassendi, Francisco Suarez and Robert Boyle.
On the other hand, occasionalism completely denied the possibility of finite causation. For occasionalists such as Malebranche, God is not merely the final cause, but the only cause, of all things. There is therefore no intra-substantial or inter-substantial causation. Substances do not affect other substances nor is there any kind of reflexive causation. Leibniz’s theory of pre-established harmony certainly has in common with occasionalism the denial of inter-substantial causation.
He would agree that there is no inter-substantial causation among the finite substances of Felipe and the chow chow’s fur when Felipe pets the chow chow, but this does not mean that God is the total real cause of the compression of the chow chow’s fur. Take, for example, a guitar player. There is a harmony between mind and body such that when the player is in a state of willing his fingers to strum, the strings are in a physical state that results in their vibration.
Leibniz’s theory of pre-established harmony had four central tenets:
1) There is no change in the state of a created substance due to being acted upon by another finite substance. In other words, there is no inter-substantial causation. He has decided that inter-substantial causation requires such physical influx, but that physical influx is unacceptable. And yet, some Leibniz scholars note that there are points out which Leibniz does seem to be sympathetic to some form of inter-substantial causation.
2) Natural change in a substance is due to that substance itself. In other words, unlike the occasionalists, Leibniz believed that there is such a thing as intra-substantial causation. Expressions or causation arising from a substance’s own nature is known as “perception.” There is an isomorphic mapping of perceptions (expressions arising from the depths of its own nature) and bodily motions, such that when you will something, your body acts in accordance with your will.
3) Every created substance possesses a “blueprint.” This is a complete concept or law that lists all of its states.
4) Each blueprint conforms with the blueprints of every other created substance. In other words, each of a created substance’s natural states coheres with the natural state of every other created substance to ever exist.