had known very little about perspective.
The science of drawing things as they look from one point of view has no doubt been taught to all of you.

You know certain rules about vanishing points and can apply them in your drawing.

But you would have found it very hard to invent perspective without being taught.
I can remember drawing a matchbox by the light of nature, and very queer it contrived to become.

Medieval artists were in exactly that same case.

The artists of the ancient world had discovered some of the laws of perspective, but the secret was lost, and artists in the Middle Ages had to discover them all over again.