One of the most interesting and important axioms employed by Spinoza in the Ethics is the so-called ‘causal axiom,’ E1a4: “Effectus cognitio a cognitione causae dependet, & eandem involvit,” which is plausibly translated as “Cognition of an effect depends on, and involves, cognition of its cause.” The Latin word cognitio can also be translated as “knowledge,” if the word is understood in a broad sense that includes items that do not meet the high standards explained in E2p40s2, for example. In that passage, cognitio is applied to the first kind of knowledge, that is, random sensory perception, as well as to higher-grade cognition of the second and third kinds. In the causal axiom, the interplay among the concepts of cause and effect, involvement, and dependence here at the beginning of the Ethics partly shapes our understanding of some crucial features of Spinoza’s philosophy.