 Spinoza begins the Ethics with the definitions of causa sui (E1d1), and of substance as that which “is conceived through itself” (E1d3). Positing these reflexive definitions at the outset might have been a calculated methodological move whose aim was to bypass the challenging task of proving the legitimacy of these notions. But since Spinoza considers his definitions as not merely stipulative (see TIE §95 and Ep. 60), we may well ask for a defense of the legitimacy of self-explanations.