When creation becomes the false light of concupiscence, it becomes illusion. The supreme value that cupidity seeks in created things, does not exist in them. A man who takes a tree as a ghost is in illusion. The tree is objectively real: but in his mind it is something that it is not. A man who takes a cigar coupon for a ten-dollar bill is also in illusion. When we view life, as if the multiplicity of the phenomenal universe were the criterion of all truth, and treat the world around us as if its shifting scale of values were the only measure of our own good, the world becomes an illusion. It is real in itself, but it is no longer real to us because it is not what we think it is.
The Assent to Truth
a appears in the Thomas Merton Reader, entitled “Visions and Illusions,” pp385