Lewis' quotation should have made the thoughtful, modern reader suspicious. If we were to educate our children, as Plato suggests, to have a specific emotional reaction to a waterfall or a Beethoven symphony demanded by the intrinsic value of the waterfall or the Beethoven symphony, we would be presupposing and imposing a conception of what is good, true, or beautiful on our children rather than educating them generally and allowing them to decide according to their own aesthetic intuitions. In imposing our cultural standards on our children, one might say, we are taking away a facet of their free will, and this, according to fellows like Kant and Locke, would be holding their individuality in contempt and thwarting their freedom to make choices based off who they are, rather than choices based off what their parents want them to be.
This sort of reasoning is primarily effeminate in its refusal to adhere to absolute values. To say that there is an absolutely wicked act or an absolutely magnanimous act is to suppose that, regardless of what other opinions are, the act is either wicked or magnanimous absolutely. It is not a matter for you to decide - it simply is. Hitler was a bad person. So far, my ostensibly effeminate audience would agree. But if I were to take this absolute truth to its obvious conclusion - that anyone who says otherwise is wrong (and is either ignorant and bad himself) - my audience would squirm and backpedal and qualify. No one would suppose that aesthetic tastes (especially musical tastes) are so clear-cut as moral judgments. But just because the path to adherence to an aesthetic standard is hard to follow does not mean that the path does not exist or should be abandoned.
This sort of reasoning is also foolish in its blind hypocrisy. Fish can't walk, donkeys can't fly, and men can't stop presupposing a conception of what is good, true, and beautiful. These cultural impositions on our children are always present, and never more than today. In the pedagogical act of refusing to draw a judgment on a waterfall or a Beethoven symphony for the sake of not imposing cultural, aesthetic standards, a teacher imposes the cultural, aesthetic standard that there are no cultural aesthetic standards. This is what he teaches, unwittingly or not, to his students with his ambivalence.
Musical education, like all education, always presupposes a conception of value. A student of Derrida or Nietzsche (as most modern educators are) would probably tell you there is no intrinsic value in art or music, which is, if nothing else is, a value judgment on the art or music. But a Medieval education presupposes that mimesis of the Trinity is always intrinsically good, true, and beautiful, which is why counterpoint, symmetry, and the antiphon demand a certain emotional response.