 The abstract schema of semantics could be seen in this way: we have a language \(L\) and a class of mathematical structures, sharing a given signature \(\Sigma\). Between these mathematical realities we have just built a bridge, the notion of truth in a structure. In one direction, we circulate from sentences to structures where these sentences are true; on the other direction, we go from structures to sentences which are true in these structures. In the first case, from a set of sentences \(\Gamma\) of signature \(\Sigma\) we define \(\Mod(\Gamma )\) as the class of structures of signature \(\Sigma\) that are models of \(\Gamma\). In the second, from a structure \(\mathcal{A}\) of signature \(\Sigma\) we define \(\Th(\mathcal{A})\) (the theory of \(\mathcal{A}\) ), the infinite set of sentences which are true at structure \(\mathcal{A}\)