_Salt dissolves in water_ is a plain enough statement; but the logician prefers to have it thus: _Salt is soluble in water_.

For he says that a proposition is analysable into three elements: (1) a Subject (as _Salt_) about which something is asserted or denied; (2) a Predicate (as _soluble in water_) which is asserted or denied of the Subject, and (3) the Copula (_is_ or _are_, or _is not_ or _are not_), the sign of relation between the Subject and Predicate.

The Subject and Predicate are called the Terms of the proposition: and the Copula may be called the sign of predication, using the verb 'to predicate' indefinitely for either 'to affirm' or 'to deny.' Thus _S is P_ means that the term _P_ is given as related in some way to the term _S_.

We may, therefore, further define a Proposition as 'a sentence in which one term is predicated of another.' In such a proposition as _Salt dissolves_, the copula (_is_) is contained in the predicate, and, besides the subject, only one element is exhibited: it is therefore said to be _secundi adjacentis_.

When all three parts are exhibited, as in _Salt is soluble_, the proposition is said to be _tertii adjacentis_.
For the ordinary purposes of Logic, in predicating attributes of a thing or class of things, the copula _is_, or _is not_, sufficiently represents the relation of subject and predicate; but when it is desirable to realise fully the nature of the relation involved, it may be better to use a more explicit form.