Aristotle deals with the theory of science or scientific knowledge mainly in the Posterior Analytics, where he presents a twofold classification of principles assumed by demonstrative science: the one is given in AnPost I.10, and the other in AnPost I.2. The science assumes three sorts of undemonstrable principles: (1) axioms, occuring in more than one science and therefore called common principles; (2) definitions of the subjects and attributes of the science; and (3) existence claims, stating that subjects of the science exist. The definitions and existence claims are called proper principles, for they are peculiar only to one science. The science assumes the definitions of all its subjects and attributes, whereas it assumes the existence only for its primary subjects. In AnPost I.2 Aristotle introduces a term "hypothesis", giving it a meaning of "existence claim". After dealing with problems of classification, the paper is concerned with some historical aspects, especially those connected with hypotheses. The concluding part is dedicated to some questions of the so-called hypothesis ad hominem, as well as to difficulties of the term "postulate".