The first premise of the demonstration hinges on a distinction between natural or innate desires and desires of a more artificial or contrived variety. Examples of the first type include the desire for food, for sex, for companionship, for beauty, and for knowledge; while examples of second type include the longing for a fashionable suit of clothes, for a fast car, for Shangri-La, or to fly through the air like a bird. Precisely because desires of the second category are externally motivated or psychologically contrived, they don’t prove anything regarding the objective existence of their objects: some of them exist and some of them don’t.
But desires of the first type do indeed correspond to, and infallibly indicate, the existence of the states of affairs that will fulfill them: hunger points to the objective existence of food, thirst to the objective existence of drink, sexual longing to the objective existence of the sexual act, etc. And this is much more than a set of correspondences that simply happen to be the case; the correlation is born of the real participation of the desire in its object. The phenomenon of hunger is unthinkable apart from food, since the stomach is “built” for food; the phenomenon of sexual desire is unthinkable apart from the reality of sex, since the dynamics of that desire are ordered toward the sexual act. By its very structure, the mind already participates in truth.
So what kind of desire is the desire for perfect fulfillment? Since it cannot be met by any value within the world, it must be a longing for truth, goodness, beauty, and being in their properly unconditioned form. But the unconditioned, by definition, must transcend any limit that we might set to it. It cannot, therefore, be merely subjective, for such a characterization would render it not truly unconditioned. And this gives the lie to any attempt — Feuerbachian, Freudian, Marxist or otherwise — to write off the object of this desire as a wish-fulfilling fantasy, as a projection of subjectivity.