A substance is capable of alteration, that is of successively (in time) having accidents which are incompatible with each other. The substance at one time is in one state and at another time is in a different state. The subsequent state is called an effect when the transition from its previous state is lawful. To be lawful, the transition to the new state must be a necessary consequence of the initial state of the substance and perhaps the states of other substances as well. The sum of the states necessary to bring about the effect is the cause. The action of the cause, that is, the bringing about of the effect, is called the causality of the cause. The passion of the effect, that it is brought about by the cause, is the dependence of the effect.
Kant's rationalist predecessors, as well as the empiricist David Hume, agreed that a causal connection is a necessary connection. Hume had argued that the necessity of a change in state cannot be established empirically, and hence that there is no objective basis for the belief that there are causal connections. Central to his argument was the claim that given any putative cause, it is always possible to conceive the non-occurrence of the effect which is supposed to follow it.
The rationalists, such as Leibniz and Wolff, held that all change is subject to the principle of sufficient reason, which is known a priori. Every transition is subject to law because in order to take place at all, there has to be a reason sufficient to bring it about.
Kant held that the category of cause and effect does not apply to things in themselves, but only to appearances. The uniform application of the category to things in themselves would lead to a strict determinism, in that every alteration that takes place would be subject to laws of nature. (See the Third Antinomy.)
The application of the category to appearances requires a "schema." The schema of cause and effect is succession. Without the possibility that a substance can be in different states successively, we could not represent it has having contradictory accidents. The claim that all appearances are subject to causal law, and consequently his rejection of Hume's skepticism, is defended in the Second Analogy.
