Since we nowadays think that all a man needs for acquisition of truth is to exert his brain more or less vigorously, and since we consider an ascetic approach to knowledge hardly sensible, we have lost the awareness of the close bond that links the knowing of truth to the condition of purity. Thomas says that unchastity's first-born daughter is blindness of the spirit. Only he who wants nothing for himself, who not subjectively 'interested,' can know the truth. On the other hand, an impure, selfishly corrupted will-to-pleasure destroys both resoluteness of spirit and the ability of the psyche to listen in silent attention to the language of reality.
... each gratification points to the ultimate one, and that all happiness has some connection with eternal beatitude. Some connection, if only this: that every fulfillment this side of Heaven instantly reveals its inadequacy. It is immediately evident that such satisfactions are not enough; they are not what we have really sought; they cannot really satisfy us at all.