And yet it is commonly confessed that we do not know the essences of things. Who knows the essence even of a fly, or (till molecular mechanics go further than they have done) of a lump of sugar? We know essences, only in their highest generalities and most abstract outlines. But we do know that, or we should know nothing. We do not understand anything unless we can say roughly what it is; and that what it is is here called the quidditas or essence. Lower animals take quiddities as they find them, e.g., a dog the quiddity of its master: they may be said to know them materially, but they do not pass upon them any explicit, formal judgement. Scientists and philosophers make it their endeavour to go beyond the quidditv, which is sufficient for the plain man to know. They start from sense, but seek to transcend sense. The 'pure quiddity' which the angel intues is not the bare abstraction visible to the popular mind: it is an intuition highly concrete, full of 'content,' discerning the essential from the accidental and the appropriate from the irrelevant, yet not ignorant even of the latter: for things irrelevant from one point of view are relevant from another. Science and philosophy is an attempt to soar from a human to an angelic view of things.
Of God and His Creatures: 3.108