I. THE PRINCIPLE OF SUCCESSION

Proposition XII. No change can happen to substances except in so far as they are connected with other substances; their reciprocal dependency on each other determines their reciprocal changes of state.

Hence, a simple substance, which is free from every external connection and which is thus abandoned to itself and left in isolation, is completely immutable in itself.

Furthermore, even were this simple substance to be included in a connection with other substances, if this relation did not change, no change could occur in it, not even a change of its inner state. Thus, in a world which was free from all motion (for motion is the appearance of a changed connection), nothing at all in the nature of succession would be found even in the inner states of substances.

Hence, if the connection of substances were cancelled altogether, succession and time would likewise disappear.

Demonstration. Suppose that some simple substance, the connection of which with other substances had been cancelled, were to exist in isolation. I maintain that it could undergo no change of its inner state. The inner determinations, which already belong to the substance, are posited in virtue of inner grounds which exclude the opposite. Accordingly, if you want another determination to follow, you must also posit another ground. But since the opposite of this ground is internal to the substance, and since, in virtue of what we have presupposed, no external ground is added to it, it is patently obvious that the new determination cannot be introduced into the being.

The same differently. It is necessary that whatever is posited by a determining ground be posited simultaneously with that determining ground. For, having posited the determining ground, it would be absurd if that which was determined by the determining ground were not posited as well. Thus, whatever determining factors exist in some state of a simple substance, it is necessary that all factors whatever which are determined should exist simultaneously with those determining factors.