There exist a number of synonyms often used indiscriminately; they are: identity, sameness, equality, congruity, similarity, and likeness. By “identity” we generally understand a sameness in every respect, absolute sameness; by “equality”, a sameness that can be expressed in figures. Equality is always a measurable sameness, and refers to quantity, mass, size, length, height, age, etc. Likeness and similitude are samenesses of form or of proportion, albeit not of size. It is often used as a partial sameness of impressions, not so much as they are in themselves, but as they appear to the mind. Congruity is a synonym of sameness in the province of geometry, denoting the coincidence of figures when laid upon one another.[10] The logical principle of identity, so-called, it appears to me, ought to be named the principle of sameness, for it has not reference to the absolute sameness of a thing with itself.[11] The statement _A_ = _A_ does not mean that this particular thing _A_ is itself and that therefore the one _A_ is one and the same thing. It is a general statement and means that all _A_, in so far as they are _A_, are the same. The statement _A_ = _A_, as I take it, presupposes the existence of a number of _A’s_; otherwise it would have no sense, and it would not only be empty, (as we know from Kant that all formal statements are,) but meaningless and useless. It would be of no avail either in logic or in science. In consideration of the fact that the idea of sameness is a fundamental concept in our scientific, logical, and philosophical reasoning, it is astonishing that no satisfactory definition of it is to be found. To define “same” as “one in substance; not other, ... of one nature or general character, of one kind, degree, or amount,” as is done in the “Century Dictionary,” is no improvement upon “Webster,” who defines it as “not different or other; identical. Of like kind, species, sort, dimensions or the like; not different in character or in the quality or qualities compared; ... like.” However, dictionaries are not encyclopædias; and they have perhaps a right to define same as identical, and identical as same. Mr. James Ward, in the “Encyclopædia Britannica,” (XVI, 81, in his excellent article on “Psychology,”) incidentally complains about the ambiguity of the word “same”; he proposes a distinction between “material identity” and “individual identity,” but this does not solve the difficulty. Flemming’s “Vocabulary of Philosophy” (4th ed. edited by Calderwood) contains several articles on “identical” and on “identity” without discussing in any one of them the meaning of “same” or of “identical.” What then is the meaning of same?