Two passages from C. S. Lewis as Philosopher: Truth, Goodness and Beauty:
The great classic triumvirate of Truth, Beauty and Goodness is a particularly apt framework for engaging C. S. Lewis and philosophy. These magnificent ideals are not only at the heart of the classic philosophical enterprise, the tradition into which Lewis was initiated in his Oxford philosophical training, but they are also of crucial significance in the Christian vision of reality he came to embrace.
From the essay “Introduction: Jack of the Philosophical Trade” by Jerry L. Walls (p. 17).
God has not left himself without witness in any of the three distinctively human, more-than-animal powers of the soul, the three aspects of the image of God in us: the mind, which knows and understands the good; the will, which chooses and enforces it; and the emotions, which love and appreciate it. This threefold structure of the soul is also the reason why so many great classics in our literature have three protagonists corresponding to these three psychological faculties and social functions: prophet, king and priest.
…The order of these three transcendentals of truth, goodness and beauty is ontologically founded. Truth is defined by Being, for truth is the effulgence of Being, the revelation of Being, the word of Being. Truth is not defined by consciousness, which conforms to Being in knowing it. Goodness is defined by truth, not by will, which is good only when it conforms to the truth of Being. And beauty is defined by goodness, objectively real goodness, not by subjective desire or pleasure or feeling or imagination, all of which should conform to it. However, the psychological order is the reverse of the ontological order. As we know Being through first sensing appearances, so we are attracted to goodness first by its beauty, we are attracted to truth by its goodness, and we are attracted to Being by its truth. But ontologically, truth depends on Being, goodness on truth, and beauty on goodness. Truth is knowing Being. Goodness is true goodness. And the most beautiful thing in the world is perfect goodness.
From the essay “Lewis’s Philosophy of Truth, Goodness and Beauty” by Peter Kreeft (p. 24-25).