C. What the Fitting Definition of a Sacrament is

207. About the third main point [n.180] I say that, in the way there can be a definition of a sacrament (as is plain from the second article, n.204), the definition is as follows: ‘A sensible sign efficaciously signifying, by divine institution, the grace of God or a gratuitous effect of God, that is ordered to the salvation of man as wayfarer’. And in the term ‘efficaciously’ is included both ‘with certitude’ and ‘with prediction’, where ‘with prediction’ is meant not only what is prior to the thing signified in duration but also prior to it in nature.

208. The proof is a universal one for things that are able to have a definition. For an account that expresses what is meant by a name is the same as a true definition, because, according to the Philosopher in Metaphysics 4.7.1012a23-24, ‘the account that a name signifies is a definition’. And this is plain through reason, because a name is imposed to signify the essence of a thing; so a statement that expresses distinctly and in its parts what is meant by a name also expresses distinctly the concept of the essence of the thing. Now the definition that has been set down, ‘A sensible sign     etc .,’ [n.207], is an account of the name ‘sacrament’ as assumed above from the use of those who speak of sacraments [n.195]; and a sacrament can have a definition in the way that beings of reason are defined. Therefore      this account is the definition of it; and that it is so is therefore proved.

209. But how it could be the definition is easily made plain, because in that account is posited something pertaining to the genus of relation, which is the genus in the sense of genus in which, in a relation of reason, genus and species are posited (namely the term ‘sign’), and the account is determined by differences in this genus (namely the genus ‘sign’), which are ‘by institution’ and ‘efficaciously’. Two things are also added there, as is universally the case in definitions of relations and relatives: one as subject or foundation, which is understood by the term ‘sensible’, and the other too, namely the correlative, which is noted by the terms ‘grace’ and ‘gratuitous’.