“‘The same’ means (1) that which is the same in an accidental sense, e.g. ‘the pale’ and ‘the musical’ are the same because they are accidents of the same thing, and ‘a man’ and ‘musical’ because the one is an accident of the other; and ‘the musical’ is ‘a man’ because it is an accident of the man. (The complex entity is the same as either of the simple ones and each of these is the same as it; for both ‘the man’ and ‘the musical’ are said to be the same as ‘the musical man’, and this the same as they.) This is why all of these statements are made not universally; for it is not true to say that every man is the same as ‘the musical’ (for universal attributes belong to things in virtue of their own nature, but accidents do not belong to them in virtue of their own nature); but of the individuals the statements are made without qualification. For ‘Socrates’ and ‘musical Socrates’ are thought to be the same; but ‘Socrates’ is not predicable of more than one subject, and therefore we do not say ‘every Socrates’ as we say ‘every man’.
“Some things are said to be the same in this sense, others (2) are the same by their own nature, in as many senses as that which is one by its own nature is so; for both the things whose matter is one either in kind or in number, and those whose essence is one, are said to be the same. Clearly, therefore, sameness is a unity of the being either of more than one thing or of one thing when it is treated as more than one, ie. when we say a thing is the same as itself; for we treat it as two.
“Things are called ‘other’ if either their kinds or their matters or the definitions of their essence are more than one; and in general ‘other’ has meanings opposite to those of ‘the same’.
“‘Different’ is applied (1) to those things which though other are the same in some respect, only not in number but either in species or in genus or by analogy; (2) to those whose genus is other, and to contraries, and to an things that have their otherness in their essence.
“Those things are called ‘like’ which have the same attributes in every respect, and those which have more attributes the same than different, and those whose quality is one; and that which shares with another thing the greater number or the more important of the attributes (each of them one of two contraries) in respect of which things are capable of altering, is like that other thing. The senses of ‘unlike’ are opposite to those of ‘like’.
“The term ‘opposite’ is applied to contradictories, and to contraries, and to relative terms, and to privation and possession, and to the extremes from which and into which generation and dissolution take place; and the attributes that cannot be present at the same time in that which is receptive of both, are said to be opposed,-either themselves of their constituents. Grey and white colour do not belong at the same time to the same thing; hence their constituents are opposed.
“The term ‘contrary’ is applied (1) to those attributes differing in genus which cannot belong at the same time to the same subject, (2) to the most different of the things in the same genus, (3) to the most different of the attributes in the same recipient subject, (4) to the most different of the things that fall under the same faculty, (5) to the things whose difference is greatest either absolutely or in genus or in species. The other things that are called contrary are so called, some because they possess contraries of the above kind, some because they are receptive of such, some because they are productive of or susceptible to such, or are producing or suffering them, or are losses or acquisitions, or possessions or privations, of such. Since ‘one’ and ‘being’ have many senses, the other terms which are derived from these, and therefore ‘same’, ‘other’, and ‘contrary’, must correspond, so that they must be different for each category.
“The term ‘other in species’ is applied to things which being of the same genus are not subordinate the one to the other, or which being in the same genus have a difference, or which have a contrariety in their substance; and contraries are other than one another in species (either all contraries or those which are so called in the primary sense), and so are those things whose definitions differ in the infima species of the genus (e.g. man and horse are indivisible in genus, but their definitions are different), and those which being in the same substance have a difference. ‘The same in species’ has the various meanings opposite to these.
Metaphysics by Aristotle – Book V