The paper deals with the main categories used in psychology for explanation of the nature of Psychics. The role of a hypothesis in the development of scientific knowledge is considered as well as the most widely spread schemes and types of explanation: the explanation of the unknown through the known; the genetic, structural and the functional explanation; the model and the probability explanation, «the explanatory sketches». The author gives especial attention to the history of the development of the principle of determinism in psychology. Besides perspectives of solution of the problem of determination of Psychics on the basis of Marxist methodology and Spinoza’s philosophical ideas are observed.