We have already suggested that science is a matter of fitting a fact or a law into a system of concepts where the concepts themselves appear to be part of a system that in turn seems to be integrated into the ') facts. The concept acts as the mediating factor between fact and science. ln legal discourse concepts have a similar role. They mediate between the facts of a situation and the rules that are relevant in such a situation. lndeed legal norms - the' ought' dimension of a rule – are incapable of direct perception, as Villa has noted, 'without the aid of concepts and theoretical categories'< Concepts and categories are thus fundamental aspects of legal knowledge. Their understanding, however, is more complex. Legal concepts can certainly be studied both diachronically and synchronically and it is the purpose of this present chapter to attempt to do just this. Yet, as we have seen with Roman legal reasoning, the way in which legal concepts relate to, and function within, facts is often part and parce! of the construction of those facts themselves. One needs therefore to distinguish between those concepts - 'institutions' that are fundamental to the actual building of the factual situation - and those concepts whose role is to inject into the legal situation a normative dimension. This is the distinction between institutions and concepts. However, it must be borne in mind that in many ways one is making a taise distinction, since it could be said that an 'institution' is simply a species of concept.