The reason that this material carries the name of the critique of “pure knowledge” is the fact that a problem is being considered here, for the solution of which neither scientific, nor philosophical approach can be applied, based on the experience of sensations and thinking of the thinking being, that is, the educated person on ideas about the knowledge of classical philosophy and science. The critique of “pure knowledge” is based on the method of transcendental materialism. It is designed to create the conditions for a clear statement of the fact that it is not acceptable for a thinking being to conceive a certain “true knowledge”, besides that knowledge, which it can comprehend in its judgment about observable signs of regular changes in states and properties of objects interacting with it, in the experience of sensations and thinking. Therefore, the subject of “pure knowledge” is all knowledge about the interaction of signs of changes in the states and properties of objects of concrete existence, accessible to observation in the experience of sensations and thinking of the “object of the living”. Moreover, in the thinking of the thinking being, “pure knowledge” can be transformed into “scientific knowledge” or “ideological knowledge”. Therefore, all other ideas about “knowledge”, the signs of which can’t be observed in the experience of sensations and thinking of the thinking being, can be defined as “imaginary knowledge”. These ideas can be based on the varying degree of diversion of thought from experience in its ability of imagination and fantasy. As a result of an extended period of “accumulation” by memorizing and recalling in experience information containing signs of “pure knowledge” in the thinking of a thinking being, signs of some kind of “new” understanding of the stored features of “pure knowledge” can appear. This phenomenon can create conditions for the formation in thinking of a creature of a thinking illusory opinion about the possibility of formulating the concepts of “pure knowledge” a priori, to the experience of sensations and thinking.