Spinoza's Ethics: II.A1: Human Essence

This first axiom of Book II is quite dissimilar to the axioms of Book I. Those axioms were basically logical building blocks. This axiom does not operate as a simple logical concept. In fact, it could be even be a proposition derived by geometric reasoning from the propositions in Book I. So, given Spinoza's rigor and brevity, this axiom and the definitions allow Book II to stand on its own grounding separate from but within the context of Book I.

Hominis essentia non involvit necessariam existentiam hoc est ex naturæ ordine tam fieri potest ut hic et ille homo existat quam ut non existat.

Translated as,

The essence of a human does not involve necessary existence, that is, from the order of nature it can occur that this and that human might exist as much as this and that human might not exist.