There are at least two reasons for the wide spread of CAI: 1. that the student is able to control his own process of learning due to immediate evaluation of his work and progress and 2. that evaluation is homogeneous, i.e. independent of subjective fac- tors, which compensates for possible lack of depth. Also, the psychology of man is such that he is less ashamed to be reprimanded for his errors by a machine than by another human being. It is (...) a specic feature of computer aided instruction of logic, particulary the art of proving, that such a system cannot be made other than open. Fortunately, in the case of logic its language makes such a creation possible. An open system is a system that permits arbitrary proof of theorems as long as the former conform to the requirement of a given formal language. The discussed system, Mizar-MSE, evolved, roughly speaking, the above line of thought. It is a natural deduction system practically applied to student instruction. Here I would like to present some of more specic features. (shrink)