9 He who reads to an audience pronounces aloud the words he sees before him: he who teaches reading, does it that others may be able to read for themselves. Each, however, communicates to others what he has learned himself. Just so, the man who explains to an audience the passages of Scripture he understands is like one who reads aloud the words before him. On the other hand, the man who lays down rules for interpretation is like one who teaches reading, that is, shows others how to read for themselves.
So that, just as he who knows how to read is not dependent on some one else, when he finds a book, to tell him what is written in it, so the man who is in possession of the rules which I here attempt to lay down, if he meet with an obscure passage in the books which he reads, will not need an interpreter to lay open the secret to him, but, holding fast by certain rules, and following up certain indications, will arrive at the hidden sense without any error, or at least without falling into any gross absurdity.
And so although it will sufficiently appear in the course of the work itself that no one can justly object to this undertaking of mine, which has no other object than to be of service, yet as it seemed convenient to reply at the outset to any who might make preliminary objections, such is the start I have thought good to make on the road I am about to traverse in this book.
Source: Christian Doctrine (New Advent)