Clearly, then, no science can interpret the fixity of terms quite literally. Or rather, it can only interpret it literally--as a matter of the literal integrity of the _words_ that _may_ convey a meaning. But in a scientific inquiry the convention of formal logic must be reversed; the fixity of terms must be _understood_ not to be absolute, but to be merely _ad hoc_ and sufficient to convey a definite meaning, which it is desired to develop. Accordingly it must always be assumed that the results of an inquiry are to modify its terms, and that it is permissible, and indeed inevitable, to develop their meaning, so long as they remain capable of expressing and conveying the new truth. We must come to every inquiry with a willingness to learn and to expand our terms. The Fixity of Terms, as it is tacitly presupposed in the traditional logic, is a scientific blunder of the gravest kind. § 13. (3) To renounce it, however, entails further consequences. It appears to undermine the whole notion of _formal validity_. For if we admit in principle that the meaning of terms depends vitally on that of the judgement in which they occur, how can we continue to rely absolutely on the mere verbal identity of its terms to hold together a syllogism? In any syllogism the middle term, _M_, may have one shade of meaning in relation to _P_, another in relation to _S_. It may be quite right to call _M_ _P_ in one connexion, and to call _S_ _M_ in another; and yet, when the two assertions are put together, they may lead to a conclusion which is an error or an absurdity. The man who (in his laboratory) would rightly declare that ‘all salt is soluble in water’ and (at his dinner table) as properly hold that ‘all Cerebos is salt’, could not combine these assertions to draw the conclusion that ‘all Cerebos is soluble in water’, without finding that the facts confuted his anticipation.