The Scotists within what, as a thing, is undifferenced, profess to find actually different "realities," which they also call "formalitates." . . . The individual man, Peter, is one undifferenced object, yet the individuality, considered formally as the individuality is not the humanity considered formally as the humanity. Hence the Scotists argue that there must be some real difference between them a parte rei, in the object itself: it need not be a difference between thing and thing, but at least it is a difference between a real formality and another real formality in one thing. Their opponents deny that the conclusion follows from the premises: they affirm that our method of abstracting one aspect from another is such, that two different aspects can be taken of an object which in itself presents no real distinction of its own, to correspond with that which we mentally make. Of itself it offers to the mind a ground for drawing the distinction, but it does not do more. There is then a virtual distinction, but there is not an actual one. This explanation seems intelligibly to meet all the requirements of the case: whereas the Scotist distinction between res and realitas is an enigma, which its proposers have no right to force upon our acceptance. Either they mean no more than our explanation admits, or if they do mean more the addition is not acceptable. For it would drive us to suppose, that whenever the weakness of our intelligence obliges us to conceive an object by a succession of ideas, one of which does not include the notes contained in another, there we come across some actual distinction in the object conceived. A doctrine which fits in better with a sound system of philosophy is, that what in itself is undistinguished is to us distinguishable by mental abstraction.
This is a pretty fair account of the formal distinction and sounds like a pretty fair critique. The problem that I have with it is twofold:
1) First, the notion of the "undistinguished in itself" which nevertheless provides a "ground" for the distinction of reason, which in the thing remains "virtual", is specious. Either Socrates' humanity and Socrateity are in every respect absolutely identical, or they are not. If so, what is the "ground" in the thing for distinguishing between them in abstraction? If not, and if all agree that Socrates' humanity and his individuating factor cannot be separated and are not really distinct, then we need some intermediate distinction.
2) I deny that the Scotist distinction between res and realitas is an enigma. On the contrary, it is quite clear. Socrates is one and self-identical. Socrateity and humanity in Socrates are not altogether and in every respect the same. Socrateity is of itself individual; humanity is common. Socrateity exists only in Socrates; humanity exists both in Socrates and in Plato. While it is the case that humanity is inseparable from Socrateity, in the sense that this particular instance of humanity cannot exist apart from Socrates, because Socrates without this humanity is not Socrates, just as Socrates without Socrateity is not Socrates, nevertheless humanity as a common nature, as existing both in Socrates and in Plato - and it does not belong to humanity as common and as a specific formal ratio to belong to Socrates, but only insofar as it is also a this, which is outside its formal ratio and provided precisely by the additional determination of its individuating factor - it can and does exist outside of Socrates. Therefore this really existing humanity in Socrates and the individuating formal factor in Socrates cannot be separated in reality, and yet they are not wholly identical, but are distinct to the extent just explained, and so are distinct in this sense prior to any consideration by the intellect. So they are formally distinct.
Similarly, the poem the Iliad is a single intelligible matter (this collection of words) with a single intelligible form (this arrangement of those words). Within this poem many formal realities can be distinguished, examined individually, and considered apart from one another. For instance, the character of Achilles is not the same thing as the plot of the poem as a whole; neither is the same as the style of the poem; nor are any of these identical with the hexametric rhythm. All of these - the character of Achilles, the plot, the style, the rhythm - pervade the poem and are in some sense present in all its parts (Achilles' character is present throughout, for instance, as the (proximate or remote) efficient and final causes of most of the action, even when he's not onstage). None of these elements are really distinct from these words in this order, nor consequently from each other. However, they are clearly not all absolutely identical with each other either. None of them could be removed from the Iliad without destroying the poem; but any of them can exist somewhere else without the others. As this, as actually existing in this poem, they necessarily coeexist; as considered as formal ratios in themselves, they need not necessarily coeexist. For instance, the style is almost inevitably lost, along with the hexametric rhythm, in a translation which retains the plot and the character. Or a new poem could be written containing the character of Achilles, but not the plot; or the same plot could be recycled with different characters, and so on. This clearly shows that the distinction between these different elements is not purely a product a product of my mind, but rather my mind's distinguishing follows from what in the poem is already distinct.
We don't posit the formal distinction, then, because the "weakness of our intellect obliges" us to conceive of things as different which are really inseparable; but because our intellect grasps the different realities which, although in the thing as individually existing are really and inseparably identical, are not wholly identical and may in some circumstances, in another individually existing thing, exist apart.
In the main text Hickey, after giving another summary account of the formal distinction, says, along with a quote from someone else named Liberatore (I'm clearly not completely up on my manualist writers):
Atvero invenire . . . quamdam tertiam distinctionem, subtilius est quam quod intelligi possit. Porro, "haec opinio . . . est vana et periculosa. Est vana, quia ad distinguenda ea, propter quae adstruitur, sufficit distinctio rationis cum fundamento in re. Est autem periculosa, quia, quum istae formalitates a natura rei proponantur ut totidem distinctae perfectiones, officit simplicitati divinae naturae."
And they find a certain third distinction, unthinkably subtle. Yet "this opinion is vain and dangerous. Vain, because a distinction of reason with a foundation in the thing is sufficient to distinguish those on account of which it is added. Dangerous, because, since those formalities are proposed as being on the side of the nature of thing, as so many distinct perfections, it impedes the simplicity of the divine nature."
Of course, from our point of view, and as we have argued here and elsewhere many, many times, the beauty of the formal distinction is precisely that, without positing any composition whatsoever in God, it serves to render meaningless any difficulties that might arise from positing an absolute identity between, say, the intellect and will in God. Because the formalities are not really distinct - they are not, for instance, different parts - they don't detract from perfect and complete simplicity. But because they do not formally include one another and so are not absolutely and in every respect identical, it is possible, for instance, that God understands something which he does not create, or understands necessarily what he creates contingently.
More on the formal distinction, among other places, here.