Mark 4:2

(ubi sup.) For He rouses the minds of His hearers by a parable, pointing out objects to the sight, to make His discourse more manifest.

A parable is a comparison made between things discordant by nature, under some similitude. For parable is the Greek for a similitude, when we point out by some comparisons what we would have understood. In this way we say an iron man, when we desire that he should be understood to be hardy and strong; when to be swift, we compare him to winds and birds. But He speaks to the multitudes in parables, with His usual providence, that those who could not take in heavenly things, might conceive what they heard by an earthly similitude.

We try to think in a general way about every parable, the elaborate interpretation of which has not been recorded by the Evangelists, remembering that Jesus explained all things to his own disciples privately. The writers of the Gospels have withheld any detailed exposition of the parables, because the things signified by them were beyond the power of words to express. Not even the whole world itself could contain the books that might be written to fully clarify and develop the parables. But it may happen that a receptive heart will grasp something of them. Purity of intent enables greater discernment of the parables, that they might become written on the heart by the Spirit of the living God. But someone might then reply that we act with impiety when we want to give the parables symbolic meaning, as if we had the authority to expound what is secret and mystical. This is sometimes claimed even in cases where one might suppose that we had some reasonable knowledge of their meaning. But to this we must respond that, if there are those who have obtained some gift of accurate apprehension of these things, they know what they ought to do. But as for us, we readily acknowledge that we fall short of the ability to see into the depth of the things here signified. We do better not to venture to commit to writing those things of which, even after much examination and inquiry, we have only some crass perception, whether by grace or by the power of our own minds. But some basic things, for the sake of our own intellectual discipline, and that of those who may chance to read them, we are permitted to some extent to set forth.