These are sometimes called one in this way, but sometimes it is the higher genus[1] that is said to be the same (if they are infimae species of their genus) — the genus above the proximate genera,[2] e.g. the isosceles and the equilateral are one and the same figure because both are triangles, but they are not the same triangles.[3]
(d) Two things are called one, when the formula which states the essence of one is indivisible from another formula which shows the essence of the other (though in itself every formula is divisible). Thus even that which has increased or is diminishing is one, because its formula is one, as, in the case of planes, is the formula of their form. In general those things, the thought of whose essence is indivisible, and cannot separate them either in time or in place or in formula, are most of all one, and of these especially those which are substances. For in general those things that do not admit of division are one in so far as they do not admit of it, e.g. if something qua man does not admit of division, it is one man; if qua animal, it is one animal; if qua magnitude, it is one magnitude. — Now most things are called one because they do or have or suffer or are related to something else that is one, but the things that are primarily called one are those whose substance is one, — and one either in continuity or in form or in formula; for we count as more than one either things that are not continuous, or those whose form is not one, or those whose formula is not one.
(e) While in a sense we call anything one if it is a quantity and continuous, in a sense we do not unless it is a whole, i.e. unless it has one form; e.g. if we saw the parts of a shoe put together anyhow we should not call them one all the same (unless because of their continuity); we do this only if they are put together so as to be a shoe and have already some one form. This is why the circle is of all lines most truly one, because it is whole and complete.
