The essay studies four aspects of Buridan’s use of principles in his logic. (1) Some principles, such as being said of all or of none, are taken over from the tradition but applied with some qualifications: for example, certain modal syllogisms governed by this principle are said to be “quasi-perfect.” (2) Some principles not used by Buridan’s realist predecessors are introduced, for example, Things that are identical with one and the same thing are identical with each other. (3) Some principles familiar from the tradition are put to powerful new uses, for example, In every pair of contradictories, one is true and the other false, and it is impossible for both to be true together or for both to be false together; again, every proposition is true or false, and it is impossible for the same proposition to be true and false together. (4) Some principles, taken from Porphyry, the Categories, and the Posterior Analytics, which were used by realist logicians to connect the theory of genus and species with the theory of modal syllogisms, are no longer used in that way. It seems that (1)–(3) have the effect of making the theory of consequences (and thus the syllogistic) more “scientific,” while (4) has the effect of separating the theory of consequences from the logical theories contained in the books of the Organon other than the Prior Analytics. If this is so, then Buridan’s logic marks an important stage in the emergence of modern conceptions of logic. A further question is how all of this relates to Buridan’s professed humanistic conception of logic as an art – not a science – “just as the leader is the savior of the army, so reasoning with learning is the leader in human life, whether that life be contemplative, namely, speculative, or active.”