The article analyzes the concepts of «unity» and «element» of psychological systems used by Vygotsky. Five signs of «units» and «elements» of the system has been described. These signs are logically symmetrical because their wording transformed into each other by denying or replacing certain words reversed. Some concepts of the theory of psychological systems considered and some differences between the concepts of «unit», «minimal unit» and «cell» of psychological system were described.

Psychological systems are defined as a holistic, sensible and active processes and/or outcomes of human interaction with the world. Examples of such systems are «the system of psychological functions» (according to Vygotsky), as well as purposeful human actions and operations (the interaction with the world at the level of objects and means). The concept of «component» of the psychological system is defined as any «something» that is in some sense belongs to human interaction with the world or is included in this interaction The component belonging to the system is called its element, but a component included in the operation and development of the system is called a part of it. The mathematical justification of these definitions is given. Identified and discussed the substantial (independently existing ) components of psychological systems and their components attribute ( properties and relations ). The relationship between bipolar theoretical constructs «part - element « and «substantial - attributive» component systems are described.