  According to Fleck, the basic phenomenon we have to accept in order to build the edifice of epistemology is the collective mental differentiation of men (1936, II). People perceive and think differently, but those differences concern groups rather than individuals. So we speak of various ways of perceiving and thinking about stars and planets by astronomers, by astrologists, by theologians or by mystics. Those people understand each other very well within their groups: an astronomer understands another astronomer well, and an astrologist another astrologist; but between groups—like between an astrologist and an astronomer—there is usually no understanding.