Abstract
A teacher of mathematics must not only master solution of mathematical problems, their selection and analysis, but also be able to introduce them to the pupils in such a way so as to stimulate the pupils' thinking processes. A priori analysis becomes a professional tool that helps the teacher in the process of this selection and decision making.
The paper analyses the differences between what is included in a priori analysis in the Theory of Didactical Situations in Mathematics on the one hand and reality of every day teaching on the other.