The doctrine of Proof, by which, starting from what is known and well-established, we arrive at a knowledge of something formerly unknown, is, according to Farabi, Logic properly so called. Acquaintance with the leading Concepts (the Categories), and with their synthesis in Judgment (Hermeneutics) and in Inference (First Analytics) furnishes only the introduction thereto. And in the Proof-doctrine the chief point is to ascertain the Norms or principles of a universally valid and necessary Science, which Philosophy has to be. Here the Law of Contradiction is looked upon as the highest of these principles, by which law the truth or necessity of a proposition, and at the same time the untruth or impossibility of the contrary, become known in one single cognitive act. From this point of view the Platonic Dichotomy is to be preferred, as a scientific method, to the Aristotelian Polytomy. And Farabi is not content with the formal side of the doctrine of proof. That doctrine has to be more than a methodology which points out the right way to the truth: it must itself point out the truth; it must generate science. It not only deals with judgments as material for the syllogism, but it enquires also into the truth which they contain, with reference to the particular sciences concerned. It is not a mere implement; it is rather a constituent part of philosophy. As we have seen, the theory of proof terminates in necessary knowledge, corresponding to necessary existence. But besides this there is the great province of the Possible, from which we can gain only a probable knowledge. The different degrees then of probability, or the modes in which we attain to a knowledge of the Possible, are discussed in the Topics, and with them are associated Sophistic, Rhetoric and Poetics. In other connections these last three subjects are mainly concerned with practical aims, but in Farabi’s hands they are combined with the Topics into a Dialectic of the Seeming. He proceeds to say that true science can be built up only on the necessary propositions of the Second Analytics, but that Probability shades off into the mere phantom of truth, from the topical or dialectic judgments down to the poetical. Thus Poetry stands at the very bottom of the scale, being in Farabi’s opinion a lying and immoral absurdity. In the addendum to the Isagoge of Porphyry, our philosopher has also given expression to his views on the question of ‘Universals’. He finds the Particular not only in things and in sense-perception but also in thought. In like manner the Universal exists not merely as an ‘accident’ in individual things, but also as a ‘substance’ in mind. The mind of man abstracts the Universal from things, but it had an existence of its own before these. Virtually therefore the triple distinction of the ‘ante rem’, ‘in re’ and ‘post rem’ already occurs in Farabi.