Intellect of a person has been usually treated as a subsystem of a personality, i.e., as some structure, or as some static property of an individual ( such as gifts, will, or moral beliefs). Thus, intellect has been considered as a static entity related to a human being. However, intellect is always displayed in the behavior and more exactly, in intellectual activity of a person. The term intellectual activity has been extensively used by different authors for a long time, but it has not been sufficiently exact lacking an adequate and efficient definition. Consequently, this has implied different difficulties for understanding and investigation of this phenomenon.

Such an exact definition was obtained by the author. According to this definition, intellectual activity is a meaningful functioning of mind (intelligent thinking). This definition provides for the dynamic expression of human intellect as well as for elaboration of efficient means for its study. That is why, an investigation of various properties of intellectual activity is of the greatest interest to psychology and pedagogy.

Basing on essential components of human activity, different types and grades of intellectual activity are separated and investigated. Relative to the result of intellectual activity, there are three types: reproductive, bounded productive, and productive intellectual activity. Relative to the means which are used in achieving the result, there are also three types: reproductive instrumental, extended instrumental, and creative intellectual activity.

Measures and methods of evaluation of intellectual activity are considered. Applications of the obtained results to problems of education are discussed.