The limits of the functional resistance in man under various extreme conditions are not fully known though there are a great number of studies on endogenous and exogenous exposure. The question on possible human limits to extreme conditions (EC) will be unanswered until individual differences and individual adaptation possibilities determining functional flexibility to EC are not investigated. The acuteness of this problem arises on one side from the potential of the human body and from the other side by a combinatory multitude of extreme exposures, the frequent unpredictability of the degree of the exposure and the speed of its acceleration as well as an individual functional resistance.

The existence of any functional system including an ergatic system (ES) can be considered feasible if it operates steadily. Sustainability is one of the basic concepts of cybernetics closely connected with the idea of invariantness. The system can change its behavior; however, some of its properties remain unchanged. Moreover, a sustainable operating system should support certain parameters, namely invariant ones. We can suppose that in this case the system in question is invariant in regard to these parameters. Under ES personnel the people are meant that interact with the technical part of the system that is a staff supporting the system and making decisions that are required for its sustainable function. ES is complicated system focused on the human being with an indefinite behavior accounted for the presence of logic with an incomprehensive sense for reality, connections and rules for conclusions characteristic of human mentality.

The analysis of ES sustainability with the methods applied for estimation of reliability of mechanical systems (the systems based on the laws of mechanics, thermodynamics etc) is impossible. To understand and analyze such systems the apparatus of operative linguistics and the properties of indistinct (blurring) ideas and multitudes is normally used. The ES perfection can be estimated by the degree of language accuracy that provides circulation of the information in the system; that is by linguistic properties approximating the language of human mentality. One of the basics of operative linguistics is a “linguistic variable”, which differs from a mathematical variable due to multivalued representation of the information.

Creating an ES model under EC brings to understanding of a multilevel mentality model and to an interational approach in the implementation of the mental process using oral attributes, variables and algorithms which realize the network structure (as the branches of the “tree of events”) described with the help of term- multitudes and frames of decisions. The use of the apparatus of incomprehensive ideas enables to present the processes of decision making as multivalued functions (with the estimation of productivity of the corresponding decisions) resulted in defining the interational algorithm of matrix positioning at the linguistic variables imitating the process of mentality and defining the possibility of its realization by means of computer engineering.