St. Thomas says the desire for wealth in a sense is infinite, because “it is the servant of disordered concupiscence, which is not curbed …”.
He is responding to the argument that man’s happiness consists in wealth. The argument starts with the premise that happiness consists in the sovereign good. The desire for the sovereign good is infinite. Since the desire for wealth is infinite, perhaps wealth is the sovereign good, and therefore that in which happiness consists.
St. Thomas replies:
‘[T]he desire for artificial wealth is infinite, for it is the servant of disordered concupiscence, which is not curbed, …. Yet this desire for wealth is infinite otherwise than the desire for the sovereign good. For the more perfectly the sovereign good is possessed, the more it is loved, and other things despised: because the more we possess it, the more we know it. Hence it is written (Sirach 24:29): “They that eat me shall yet hunger.” Whereas in the desire for wealth and for whatsoever temporal goods, the contrary is the case: for when we already possess them, we despise them, and seek others: which is the sense of Our Lord’s words (John 4:13): “Whosoever drinketh of this water,” by which temporal goods are signified, “shall thirst again.” The reason of this is that we realize more their insufficiency when we possess them: and this very fact shows that they are imperfect, and the sovereign good does not consist therein.’
The desire for wealth and the desire for the sovereign good are both infinite, in that we can never get enough of either — but for different reasons. We can’t get enough of temporal goods because they never satisfy. We want some physical thing, but when we get it the novelty quickly wears off and we want something else. “Whoever drinks of this water will thirst again.” Whereas the reason we can’t get enough of the sovereign good is the more we have it, the more we love it, and the more we despise other things in its favor. It’s both satisfying and inexhaustible.
There’s no physical thing that’s both satisfying, causing us to despise all other things, and inexhaustible. Therefore the sovereign good is spiritual.
Thus, the spiritual is higher than the physical.