The third book of Locke's Essay is upon words and language; and in the order of treatment this would be the next topic for discussion. But much of what is said in this connection both by Locke and by Leibniz is philological, rhetorical, and grammatical in character, and although not without interest in itself, is yet without any especial bearing upon the philosophical points in controversy. The only topics in this book demanding our attention are general and particular terms; but these fall most naturally into the discussion of general and particular knowledge. In fact, it is not the terms which Locke actually discusses, but the ideas for which the terms stand. We pass on accordingly, without further ceremony, to the fourth book, which is concerning knowledge in general. Locke defines knowledge as "nothing but the perception of the connection and agreement, or disagreement and repugnancy, of any of our ideas." These agreements or disagreements may be reduced to four sorts,--Identity, or diversity; Relation; Co-existence, or necessary connection; Real existence. The statement of identity and diversity is implied in all knowledge whatsoever. By them "the mind clearly and infallibly perceives each idea to agree with itself and be what it is, and all distinct ideas to disagree; _i. e._, the one not to be the other." The agreement of relation is such knowledge as the mind derives from the _comparison_ of its ideas. It includes mathematical knowledge. The connection of co-existence "belongs particularly to substances." Locke's example is that "gold is fixed,"--by which we understand that the idea of fixedness goes along with that group of ideas which we call gold. All statements of fact coming under the natural sciences would fall into this class. The fourth sort is "that of actual and real existence agreeing to any idea."