This paper studies the first two questions of John Duns Scotus’s Commentary on Porphyry’s Isagoge. In these questions, Scotus establishes the scientific character of logic against the Aristotelian objection which defines logic as method of science. And, against the Aristotelian definition of a scientific object as a determined genre, Scotus establishes logic as a common science. The Spanish traduction of these two questions is presented as appendix.