The probability concept is not simple: It can be disolved into many components and we shall describe some of them. From the general point of view, the student does not acquire all these components simultaneously, but rather crosses one treshold of the other. Hence, he/she always possesses but a partial understanding. He/she will be able to answer some questions which are compatible with the tresholds already attained; but he/she will systematically fail in other tests. Besides, the mere crossing of treshold is not sufficient. fixation is indispensible to secure the knowledge acquired. Otherwise, the pupil can be asked diverting questions which reveal the instability of the knowledge acquired. This paper is a personal view of a study of Jesus ALARCON (Mexico), who presented his doctoral thesis in Strasbourg in Juni, 1982. This research is not in a genetical perspective, as this would require comparing the results of children from different age groups. The study used approx. 300 12- to 14-year -old pupils who were distributed to 3 samples of approx. equal size, i.e., 3 questionnaires were presented with slight alterations of the items to be tested. The results of only one of these samples (106 pupils) will be discussed here.