Spinoza's Ethics: III.P5 - Self-Contradiction

Res eatenus contrariæ sunt naturæ hoc est eatenus in eodem subjecto esse nequeunt quatenus una alteram potest destruere.

Things are contrary to an extent, that is, their natures are unable to exist thus in the same subject to the extent that one might destroy the other.

DEMONSTRATIO: Si enim inter se convenire vel in eodem subjecto simul esse possent, posset ergo in eodem subjecto aliquid dari quod ipsum posset destruere, quod (per propositionem præcedentem) est absurdum. Ergo res etc. Q.E.D.

For if they are able to join among themselves or be in the same subject at the same time, thus something is able to exist in the same subject which might destroy itself which (by IIIP4) is absurd. 

Here Spinoza is being propositional - inherent self-contradictions cannot exist. This is a fundamental stance of rationalists versus empiricists. Empiricists can embrace self-contradictions, e.g. a wave can't be a particle and vice versa, yet seems to do so in basic atomic physics. Rationalists cannot as logic is the structure of a coherent universe.