My wish is to provoke a reflection on the question of time, without separating it from that of space, as the theory of relativity invites us to do; but going further into the idea of a conceptual unity (relativity keeps two distinct concepts, two types of instruments, rulers and clocks, and always considers in advance two sets of variables). A reflection of this kind cannot avoid a discussion on how our rationality works in building our knowledge of the world. In composition with the more usual substantial rationality, it is appropriate to speak of a relational rationality: inside the world, we can only make comparisons / oppositions (time is not space, space is not time) without being able to assign to one or the other a list of characters of its own. In this context, we are led to give to movement a primary character (beginning with showing it, before defining it with words). Space and time are derived from it: space is associated with relatively slower movements than those on which we define time. A list of consequences in physics will be proposed: - a look at the "speed" of light; - the twins paradox; - Lorentz transformation and the construction of time; - the relations between quantum mechanics and general relativity from the point of view of their representations of time and space; - the composition of non-collinear Lorentz transformations ... Other avenues of research emerge in humanities and social sciences.