Philosophers did not always pay attention to knowledge of the existence of beings. This mainly resulted from the fact that there is a strict correlation between how being is understood and the conception of knowledge. If existence is not considered as a constitutive fact of being, then it is not strange that one does not pay more attention to it in explaining knowledge, and conversely, the recognition of existence as a fundamental element of the structure of being cannot be not translated into a fundamental role in knowledge. The close connection of knowledge with the structure of being indicates that the explanation of cognitive acts that apprehend existence requires many systemic analyses. However, such explanations usually have not enough a systemic character, since they most often concentrate on describing and explaining cognitive acts alone, but do not sufficiently emphasize the reasons for knowledge on the side of the subject and on the side of the object. In the twentieth century the Polish Dominican Father M. A. Krąpiec made an attempt to grasp these problems in a complementary way. He was inspired by the thought of St. Thomas Aquinas and formulated an original explanation of the knowledge of the existence of beings. At the same time he indicated the basic role of this act in all metaphysical knowledge and made cognitive realism dependent on this act.