Explain "wholeness, harmony, and radiance."

This quotation comes from A Portrait of the Artist as a Young Man by James Joyce. It forms part of Stephen Daedalus's interpretation of St. Thomas Aquinas's aesthetic theory, which he patiently explains to one of his schoolmates. The great medieval philosopher set down three criteria for what is truly beautiful, which can be translated from the original Latin as wholeness, harmony, and radiance. Each of these essential qualities corresponds to a specific phase of artistic apprehension. Let's take the example that Stephen Daedalus uses, that of a basket.
The first stage of aesthetic apprehension involves isolating the object—in this case, a basket—from its surroundings. It is only then, when it has been freed from the world of space and time, that the object can be apprehended as a singular thing in its own right, in its wholeness.
If the first stage involved seeing the object, the second stage of artistic apprehension involves analyzing it. This means that one starts to examine all the features of the object—its lines, shapes, and colors—observing how all the various parts fit together to form a whole. This is what Daedalus, following Aquinas, means by an object's aesthetic harmony.
Having seen the object and analyzed it into its component parts, what we now need to do is synthesize it, or put the whole thing back together in our minds. Using our example, we are now in a position to see the basket as this thing and no other. The final stage of aesthetic apprehension involves an appreciation of what medieval philosophers called the quidditas of a thing, its essence. Once this stage has been reached, then the object's beauty stands revealed in all its radiance, illuminating the mind of the artist, stimulating their imagination, and inducing a state of supreme aesthetic pleasure.