‘By cause of itself I understand that whose essence involves existence, or that whose nature cannot be conceived except as existing.’ This is how the Ethics begins – with the affirmation (affirmatio) of an insight through which true thought and free life become possible. Causa sui is not an abstract principle and yet nothing can be derived from it; it is reality – essence identical with its power (puissance) – as ‘absolute affirmation’ or autonomy. Although Spinoza draws on a long tradition by using this term, he rejects both its most immediate and its most remote legacy. He rejects interpretations inspired by either Aristotle's final cause or Descartes’ efficient cause. However,

The expression, ‘that whose essence involves existence’, should thus help us to understand how much this inclusion of existence within essence emphasises the immanent presence of a cause or reason inherent in that essence. The presence is the infinite in acts, that is, the ‘absolute affirmation of the existence of some nature’. Infinity is thus known through the reason or cause that explains (develops and affirms) its presence. In other words, infinity is known through its inner cause; a cause that, when developed, explains everything about the harmony between the essence and existence of a thing. To be the cause of oneself is to affirm one's cause absolutely (perfectly or completely). Causa sui is the paradigm of the absolute affirmation of the cause (or reason). It is also the paradigm of infinity, freedom and eternity – concepts that are both expressions of the power of the self-caused and characteristic features of Spinozist thought.

There are two major misconceptions that must be avoided by the reader of the Ethics. First, the idea that God or Nature (Deus sive Natura) is without cause or reason (which would reduce the ontological value of the mathematical model). Second, the notion that this cause, if it exists, somehow contains – in the absence of reason – the mystery of its own origin (this is the operational limit of the Mathesis)