Ad decimum dicendum quod omnia, quae in Deo sunt, sunt una eius simplex essentia, sed ea, quae in ipso sunt unum, in intellectu nostro sunt multa, et propter hoc intellectus noster potest apprehendere unum istorum sine altero. Inde est quod in statu viae de nullo eorum possumus cognoscere quid est, sed solum an est, et contingit quod cognoscatur, an est unum eorum et non alterum; sicut si aliquis cognosceret, an sit sapientia in Deo, non autem an in ipso sit omnipotentia. Et similiter potest ratione naturali sciri an Deus sit, non tamen an sit trinus et unus.
-- Super Boethium de Trinitate, p. 1, q. 1, a. 4, ad 10
"It should be said that whatever is said of God is one with his simple essence, but that the things that are one in him are many in our intellect, which is why our intellect can apprehend one without the other. That is why in this life we can know of none of them what it is but only that it is, and it happens that one of these [attributes] can be known without knowing another; for example, one might know that God is wise yet not know that he is omnipotent. In much the same way we can know by natural reason that God is, but not that he is three and one."
-- tr. Ralph McInerny, Aquinas: Selected Writings (London, Penguin: 1998), p. 126.