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These agree in being relatives, who being used for persons, which for things, and that being used indifferently for either. Who and which have well-defined different uses: (a) they connect two co-ordinate sentences; as, I met a policeman who showed me the way; I studied geometry which I found useful. Each of these sentences could be turned into two propositions grammatically, as well as logically, independent: I met a policeman and he showed me the way; I studied geometry and it I found useful. Another use of the same nature is when the second clause is of the kind termed adverbial, where we may still resolve who and which into a personal or demonstrative pronoun and a conjunction: as, why should we condemn James who (for he, seeing that he) is innocent? why should we study phrenology which (seeing that it) is profitless ? (b) They are often used to introduce subordinate or adjectival clauses, which serve to define or explain a noun regarding which a statement is made in the principal clause; as, I saw the man who first taught me to swim; the house which he built still stands. Now, in these latter uses, who and which cannot be turned into and he, and it. The following sentence, standing alone, is ambiguous: “I re-read the book which gave me much pleasure.” This may mean either that the re-reading gave much pleasure, and in that case the sentence consists of two coordinate sentences and belongs to section (a), or it may mean I re-read the book which when formerly read gave me much pleasure. In the latter case the second clause limits or explains the object of the first and belongs to section (b). To remove such ambiguity, and the unpleasant effect arising from the too frequent use of who and which, it has been proposed by some grammarians (especially Professor Bain) always to employ that and not who or which, when the relative is used to introduce a restrictive or adjectival clause, and instead of saying “the man who hath no music in himself is fit for treasons, etc.,” “they are the books which nourish all the world,” to say, as Shakespeare says, “the man that hath, etc.,” “they are the books that nourish, etc.,” reserving who and which for such cases as are noticed under section (a).