Spinoza's Ethics: III.Post1

Spinoza begins his approach to the human body as a form of physics - just as he indicated in the preface. The implicit physics here is the unity of the human body as a complex composite. This sets up that affecting some parts increases a power of acting while affecting other parts decreases a power of acting. Crucial to both affections is that the body retains its unity.

Corpus humanum potest multis affici modis quibus ipsius agendi potentia augetur vel minuitur et etiam aliis qui ejusdem agendi potentiam nec majorem nec minorem reddunt. Hoc postulatum seu axioma nititur postulato 1 et lemmatibus 5 et 7, quæ vide post propositionem 13 partis II.

The human body is able to be affected by many things in many ways by which the power of acting itself is increased or decreased and even by others which return the power of acting itself neither more nor less. This postulate or axiom is supported by Postulate 1 and Lemmas 5 and 7, which see after IIP13.