"Perhaps it would be best to say very emphatically (with a blow on the table), 'There is an Is.' That is as much monkish credulity as St. Thomas asks of us at the start."

Without pretending to span within such limits the essential Thomist idea, I may be allowed to throw out a sort of rough version of the fundamental question, which I think I have known myself, consciously or unconsciously since my childhood. When a child looks out of the nursery window and sees anything, say the green lawn of the garden, what does he actually know; or does he know anything? There are all sorts of nursery games of negative philosophy played round this question. A brilliant Victorian scientist delighted in declaring that the child does not see any grass at all; but only a sort of green mist reflected in a tiny mirror of the human eye. This piece of rationalism has always struck me as almost insanely irrational. If he is not sure of the existence of the grass, which he sees through the glass of a window, how on earth can he be sure of the existence of the retina, which he sees through the glass of a microscope? If sight deceives, why can it not go on deceiving? Men of another school answer that grass is a mere green impression on the mind; and that he can be sure of nothing except the mind. They declare that he can only be conscious of his own consciousness; which happens to be the one thing that we know the child is not conscious of at all. In that sense, it would be far truer to say that there is grass and no child, than to say that there is a conscious child but no grass. St. Thomas Aquinas, suddenly intervening in this nursery quarrel, says emphatically that the child is aware of Ens. Long before he knows that grass is grass, or self is self, he knows that something is something. Perhaps it would be best to say very emphatically (with a blow on the table), "There is an Is." That is as much monkish credulity as St. Thomas asks of us at the start. Very few unbelievers start by asking us to believe so little. And yet, upon this sharp pin-point of reality, he rears by long logical processes that have never really been successfully overthrown, the whole cosmic system of Christendom.

  -St. Thomas Aquinas: The Dumb Ox (1933)