The purpose of this article is to discuss the problem of tacit knowledge, or to review the conditions whose satisfation makes it possible to ask a well formulated question about existence of such knowledge. The author has assumed that to approach this problem he must begin from expilcating the term 'tacit knowledge' in order to expound the consequences of the meaning so explicated. The article is made of four parts. In the first part the function of tacit knowledge is presented, and especially the validity of opinions which cannot be defined as justified true beliefs is subjected to scrutiny. In the second part the author discusses the concept of tacit knowledge as it is used by Michael Polanyi, and he pays special attention to the problem of the subject who possesses such knowledge. The third part deals with the relationship between tacit knowledge and the traditional conception of reliable knowledge. The fourth part returns to methodological issues connected with attempts to define tacit knowledge.