The Formal Distinction, Formalities, and Common Natures

From Scotus' Ordinatio I.Dist.5 P.1 Q.1 paragraph 19. In one mighty sentence Scotus lays out the foundations of the formal distinction and the doctrine of common natures:

In substantiis, quamvis in eadem realiter - etiam quamvis simplici - possint esse perfectiones multae substantiales formaliter distinctae et ibi una formalis ratio possit abstrahi ab alia remanente adhuc concretione utriusque formalitatis ad sua propria supposita (verbi gratia, licet haec sit vera 'substantia intellecta est volitiva' - ubi est praedicatio concretiva perfectionis unius substantialis de alia - tamen haec negatur 'intellectus est voluntas', quia ista significant perfectiones illas ut abstractas a se invicem, et hoc secundum proprias formalitates earum; tamen adhuc ista sic abstraca concernunt propria supposita, quia hic 'intellectus' est intellectus), accipiendo tamen substantiam sive simplicem sive compositam praecise secundum unam rationem formalem quidditativam, tantum est abstractio a supposito propriae naturae communiter, quia non sunt natae concernere aliquid alterius naturae; ideo ista abstractio est maxima.

"In substances, although in the same thing really - however simple they are - there can be many formally distinct substantial perfections, and there one formal ratio can be abstracted from another, the concretion of each formality remaining in its own supposit (for example, although this is true: 'an intellective substance is volitional' - where there is a concretive predication of one substantial perfection about another - nevertheless this is denied: "the intellect is the will', because these [i.e. 'intellect' and 'will'] signify those perfections as abstracted from each other, and that according to their proper formalities; still these things so abstracted concern their own [one and the same] supposit, because this 'intellect' is the intellect [i.e. the intellective faculty belongs to the intellective soul, but the volitive power does too]), nevertheless accepting a substance whether simple or composite precisely according to one formal quidditative ratio, still there is an abstraction from the substance of its proper common nature, because they do not in themselves concern anything of another nature; therefore this abstraction is the greatest."

Notice that in this paragraph he says "formally distinct" while elsewhere in the same question he also says "formal non-identity", leading one to think that there is not much difference between these two formulations.

Don't worry much here about the "greatest abstraction" bit, which is only explained by the succeeding paragraphs. The important point is Scotus' doctrine that two things can be really identical in the sense of absolutely inseparable--like intellect and will in the rational soul, like humanity and Socrateity in Socrates--and yet formally distinct. This means that the two formalities involved do not include one another in their conception or definition, and sometimes means that at least one of the formalities can exist without the other. Socrateity cannot exist without humanity, since it cannot be the case the Socrates is not human, but humanity can exist without Socrateity, since there can be (and are) many men that are not Socrates. Humanity in itself does not concern itself in anything belonging to another formality, so that it is indifferent to Socrateity or Platonity. And similarly, there can be no men which are not animals, but animality itself is indifferent to humanity and equinity, so that the formal ratio--the intelligible structure, as we might want to say--of animality is formally distinct from any specific difference, though in a concrete supposit it never exists without being determined by some such specific formality. (As a universal in the mind, however, it does.)