All things are judged good or bad according to their purpose (or telos, as the classical Greeks called it).

I know a good knife must be sharp, because I understand its purpose is to cut.

And because I know its purpose, I understand that the whetstone is good for the knife while its opposite would be bad.

In sum, because I understand the purpose or telos of the thing, I can know whether the quality of that thing is good or bad—and also what is good or bad for that thing.

So too is it for our intellect.

If the purpose of our intellect is truth, then it is by that standard I judge what is good or bad for my intellect.

Like a whetstone to the knife, a true great book will sharpen my mind’s understanding of reality.

For truth is the conformity of the mind to reality.

It is in obedience to this telos that we judge our study and the study of the great books in particular.

Not all great books meet this standard—as some are guides to the delineations of what is real, while others labor against it.