Diagnostics of mental disorder is based on an estimation of patient's behavior (including verbal behaviour), which is ruled by a situation expressed as a system of signs. Perception of a situation, in turn, depends on the context resulting from the previous situations, structuring personal world. So the world is not given - it is being formed while a purposeful activity: distinctive behaviors, including abnormal variants, can be explained not in terms of personalities or conditions but of situations in individual worlds. Like a text composed of words, individual world is composed of situations and each of them needs certain context to cope with ambiguity (inherent for most of sign systems due to polysemia). This context is induced by the world as a whole. If the context cannot help to interpret a situation, uncertainty can be eliminated by actions clarifying a situation. Thus, purposeful activity, skills to make predictions and corrections of one's own actions are crucial: weakness of any of them leads to the distortion of presentation of the world, to wrong evaluation of situations and, as a result, to inadequate actions fixing (in a vicious circle manner) the inadequate structure of the world.