Spinoza Ethics: if a thing could act in a different way, will it have a different essence?

Question

Ethics part 1: PROP. 26. A thing which is conditioned to act in a particular manner, has >necessarily been thus conditioned by God; and that which has not been conditioned by God >cannot condition itself to act.

Proof.--That by which things are said to be conditioned to act in a particular manner is >necessarily something positive (this is obvious); therefore both of its essence and of its >existence God by the necessity of his nature is the efficient cause (E1P25 and E1P16); this >is our first point. Our second point is plainly to be inferred therefrom. For if a thing, >which has not been conditioned by God, could condition itself, the first part of our proof >would be false, and this, as we have shown, is absurd.

Why does he think whenever the existence and essence of a thing is determined, a thing cannot act otherwise? Does he think a thing act in a different necessarily has a different essence?

Answer 1

The reason that whenever a thing is so determined, it cannot act otherwise, is the conjunction of three facts: 1) what ultimately determined it was necessarily the one substance, not something else, and 2) this substance necessarily exists, and 3) substance cannot exist with any other essence of its own than it actually has, an essence which necessitates all of substance's own action (1p17). The thing gets all its reality from substance because it is just a mode of substance, with the result that things could not have been produced by substance in any other way (1p33). I take your second question to be: if a thing acted differently, would it have to have a different essence? Of course it might act differently in the future from how it is acting now; Spinoza doesn't discuss how the thing's essence, which in some way is defined once and for all by the thing's efficient cause (5a2), allows the thing to change over time. As to whether a different essence would be needed for it to have acted differently now from how it is actually acting, he does not discuss such counterfactuals where the 'If it were acting differently...' is necessarily false. Maybe he would reject the 'If... then...' sentence as meaningless. Or he might say, 'It would have to have a different essence, but that couldn't happen.'