First of all we must distinguish the number of ways the term “the same” is spoken of. It may seem, in general terms, that the term “the same” falls into three divisions. For people are wont to call a thing the same either in number, or in species, or in genus.
In number, when there is more than one name but only one thing, such as coat and jacket; in species, when, though numerically more than one, there is no difference in species, such as man with man, and horse with horse – for such things are said to be the same in species, that is, those that fall under the same species. Similarly, too, those which fall under the same genus are called the same in genus, such as horse with man. However, the same water from the same spring might have a certain difference besides the ways mentioned; yet let this case too be placed in the same class with those that are in some way said in virtue of one species, since all such things seem to be related and similar to each other. For all waters are said to be the same in species among themselves in that they have a certain similarity, while the water from the same spring differs in nothing else except that the similarity is greater; this is why we do not separate it from those which are in some way said in virtue of one species.
What is numerically one
It is generally admitted by all men that what is numerically one seems most to be called the same. This one, too, is used being spoken of in a number of ways.
Most strictly and primary, when the term “the same” is attributed to a name or to a definition, just as when a coat is said to be the same as a jacket, and a two-footed land animal as a man; secondly, when (it is attributed) to peculiarity, just as that which is capable of knowledge to man, and what naturally is carried upwards to fire; thirdly, when from what is accidental, as the one who is sitting or is musical to Socrates. For all these mean to signify that which is numerically one.
That what we have just said is true, one may best find out from those who change the appellations. For often when we order someone to call on one of the people who are sitting by name, we change the appellation, if there happened not to understand us the person to whom we gave the order, since we consider that he will better understand us from what is accidental, and we bid him call to us the person who is sitting or conversing – obviously because we consider that the meaning is the same both by name and by what is accidental.
Bibliography: Aristotle Topics (103a.3)
Translation – text editing: George Kotsalis