  Also in the sixties, Lambek proposed to look at categories as deductive systems. This begins with the notion of a graph, consisting of two classes Arrows and Objects, and two mappings between them, \(\boldsymbol{s}: \Arrows \rightarrow \Objects\) and \(\boldsymbol{t}: \Arrows \rightarrow \Objects\), namely the source and the target mappings. The arrows are usually called the “oriented edges” and the objects “nodes” or “vertices”. Following this, a deductive system is a graph with a specified arrow: