Background. The question about nature and essence of psyche and psychical form of reflection till now is opened in spite of effort of many scientists. As a result the problem of scientific definition of psyche and its nature is being one of the most fundamental problems in psychology.

The Objective of the research is to show: a) where the complexity of solving the problem of psyche lies; b) what logic options of solving this problem that are offered in national psychology, particularly in the works of А.N. Leontev and N.I. Chuprikova, can be adduced and why they are insolvent; c) how it is possible to solve the problem of psyche and mental form of reflection using the major concepts of the reflection theory and adequacy concept of behaviour in constantly changing conditions of reality.

Design. The term “psyche” as a theoretical concept is introduced to explain the ability of living organism (opposed to lifeless) to react to external impact by various activity. Critical analysis of natural-scientific definitions of psyche in the works of А. N. Leontiev and N.I. Chuprikova is presented.

Their discrepancy and insufficient scientific background are shown. Main assumptions of the reflection theory are coined, and further definitions of non-mental forms of reflection (e.g. biochemical, physiological, neurophysiological) and also essence of psyche and specific features of psychical forms of reflection are shown. The paper presents a model that reverberates the author’s idea about the dynamics of the evolving psyche and mental processes (e.g. cognitive, emotional, volitionary) due to which mental forms of reflection and of behaviour regulation are fulfilled.

Results. The analysis of natural-scientific definitions of psyche and mental form of reflection that the national psychology dwells upon show their inconsistency and scientific insolvency. The author's version of solving the problem of psyche and determining specific features of mental form of reflection are given an account based on the major concepts of the reflection theory and the need for behaviour adequacy.

Conclusions. In order to shape the essence of psyche, specific features of mental form of reflection and features of mental processes it is necessary to distinguish forms of reflection, allocation of their functions and constraints. Reflection which can be attributed to non-mental (e.g. biochemical, physiological, neurophysiological) form, does not allow to provide behaviour adequacy in a rapidly changing conditions of reality. Based on reflection, behavior always occurs with certain delay in time. If eliminated, this drawback is provided by occurrence of a brain and a property named “psyche”, which provides mental form of reflection as reflection of the near future of objective reality.