The article presents Bentham's theory of fictionalism in the form in which it is developed in his Fragment on Ontology and Essay on Logic. It is shown that interest in Bentham's fictionalism is connected not only and not so much with the fact that it is a necessary part of his philosophical conception, but also with the fact that Bentham's method of explanation of fictitious entities inspired by Aristotle's logic and his Predicaments, modified the methodology of modern philosophy. The fictitious entities must be defined by a method of definition that Bentham invented, and termed "Paraphrasis". This involves placing the definiendum in a sentence, and then paraphrasing it by translating the whole sentence into a sentence equivalent in meaning in which the definiendum does not appear. Thus, Bentham's paraphrasis and theory of fictitious entities influenced the development of philosophical as well as legal conceptions of the 20th century.