The article traces a line in the development of epistemology, which led to the formation of the concept of tacit knowledge. The author demonstrates that the formation of this area of research is based on the solution to the problem of prerequisite knowledge found by I. Kant: a priori foundations of knowledge are such prerequisites for knowledge that are not the meaningful ideas but the forms of knowledge organization. The tacit knowledge as non-reflexive, non-explicit knowledge that exists on the periphery of cognitive activity functionally corresponds to just such a component of cognition; its significant part is made up of ontogenetically a priori prerequisites for cognition. The tacit knowledge is irreducible to the personal knowledge; on the contrary, the category of collective tacit knowledge exists. The article provides an overview of the possible sources of tacit knowledge identified in various areas of the study of the problem.