Things are said to be other in species if they are of the same genus but are not subordinate the one to the other, or if, while being in the same genus they have a difference, or if they 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 formulae differ in the infima species of the genus (e.g. man and horse are indivisible in genus, but their formulae are different), or which being in the same substance have a difference. '