The natural tendency is to attribute the same meaning to the same word wherever it occurs. We instinctively treat a language as if it were a fixed system of signs. Fixity, indeed, is a characteristic of the signs which have been expressly invented for scientific use, such as algebraical notation or the nomenclature of chemistry. Here every expression has a single precise meaning, which is absolute and invariable; it expresses an accurately analysed and defined idea, only one such idea, and that always the same in whatever context the expression may occur, and by whatever author it may be used. But ordinary language, in which documents are written, fluctuates: each word expresses a complex and ill-defined idea; its meanings are manifold, relative, and variable; the same word may stand for several different things, and is used in different senses by the same author according to the context; lastly, the meaning of a word varies from author to author, and is modified in the course of time. _Vel_, which in classical Latin only has the meanings _or_ and _even_, means _and_ in certain epochs of the middle ages; _suffragium_, which is classical Latin for _suffrage_, takes in mediæval Latin the sense of _help_. We have, then, to learn to resist the instinct which leads us to explain all the expressions of a text by their classical or ordinary meanings. The grammatical interpretation, based on the general rules of the language, must be supplemented by an historical interpretation founded on an examination of the particular case. The method consists in determining the special meaning of the words in the document; it rests on a few very simple principles. (1) Language changes by continuous evolution. Each epoch has a language of its own, which must be treated as a separate system of signs. In order to understand a document we must know the _language of the time_--that is, the meanings of words and forms of expression in use at the time when the text was written. The meaning of a word is to be determined by bringing together the passages where it is employed: it will generally be found that in one or other of these the remainder of the sentence leaves no doubt as to the meaning of the word in question.[135] Information of this kind is given in historical dictionaries, such as the _Thesaurus Linguæ Latinæ_; or the glossaries of Du Cange. In these compilations the article devoted to each word is a collection of the passages in which the word occurs, accompanied by indications of authorship which fix the epoch. When the author wrote in a dead language which he had learnt out of books--this is the case with the Latin texts of the earlier middle ages--we must be on our guard against words used in an arbitrary sense, or selected for the sake of elegance: for example, _consul_ (count, earl), _capite census_ (censitary), _agellus_ (grand domain).