Around 1891, Husserl opposes, in several articles (cf., Husserl, 1891a and Husserl, 1891b), the logic of extension, developed by Peirce, Schröder and others, and the logic of intension. But his distinction does not concern the difference between the extension and the intension of a concept. In fact, the systems based on the extensional axiom of extensionality [a=b ↔ ∀x (x ɛ a ↔ x ɛ b] or the intensional axiom of extensionality [a=b ↔ ∀x (a ɛ x ↔ b ɛ x] are equivalent, provided one presupposes the pairing axiom. Husserl has something else in mind: he thinks that both forms of extensional logic cannot reach their purposes, because a simple partial domain of deductive logic is confounded with deductive logic itself (1891b, 244).