Accumulating knowledge of the outside world, fixing data, comprehending them, the person creates difficult system of representations - a certain conceptual model which finally is fixed by a natural language. The model of the outside world at the person is poorly typified and poorly formalized, such model assumes operating images, and in it it is possible to allocate syntactic, semantic and pragmatical levels. However it is necessary to remember that it is model.
At an impracticability of direct control of operation of the machine and exhaustive check of results of machine operations, it makes sense seeks to provide nevertheless the most achievable control and the most achievable reliability of methods of check of results of computer calculations. Means of achievement of this purpose are various for various types of systems.