2 Corinthians 3:18
And let this that we have said about habit be our speech unto the young; since to those who are men and taught in heavenly wisdom, stronger than all is the fear of God, the remembrance of hell, the desire of the kingdom of heaven; for these are able to quench the fire. And along with these take that thought also, that what you see is nothing else than rheum, and blood, and juices of decomposed food. 'Yet a gladsome thing is the bloom of the features,' says one. But nothing is more gladsome than the blossoms of the earth, and these too rot and wither.
Do not then in this either give heed to the bloom, but pass on further inward in your thought, and stripping off that beauteous skin in your thought, scan curiously what lies beneath it. For even the bodies of the dropsical shine brightly, and the surface has nothing offensive; but still, shocked with the thought of the humor stored within we cannot love such persons. 'But languishing is the eye and glancing, and beautifully arched the brow, and dark the lashes, and soft the eyeball, and serene the look.'
But see how even this itself again is nothing else than nerves, and veins, and membranes, and arteries. Think too, I pray, of this beautiful eye, when diseased and old, wasting with despair, swelling with anger, how hateful to the sight it is, how quickly it perishes, how sooner even than pictured ones, it is effaced. From these things make your mind pass to the true beauty. 'But,' says he, ' I do not see beauty of soul.' But if you will choose, you shall see it: and as the absent beautiful may be with the mind admired, though with one's eyes unseen, so it is possible to see without eyes beauty of soul.
Have you not often sketched a beauteous form, and felt moved unto the drawing? Image also now beauty of soul, and revel in that loveliness. 'But,' says he, 'I do not see things incorporeal.' And yet we see these, rather than the corporeal, with the mind. Therefore it is, for instance, that although we see them not, we admire angels also and archangels, and habits of character, and virtue of soul. And if you see a man considerate and moderate, you will more admire him than that beautiful countenance.
And if you see one insulted, yet bearing it; wronged, yet giving way, admire and love such, even though they be striken in age. For such a thing is the beauty of the soul; even in old age it has many enamored of it, and it never fades, but blooms for ever. In order then that we also may gain this beauty, let us go in quest of those that have it, and be enamored of them. For so shall we too be able, when we have attained this beauty, to obtain the good things eternal, whereof may all we partake, through the grace and love towards men of our Lord Jesus Christ, with Whom to the Father, with the Holy Spirit, be glory and might, for ever and ever. Amen.
Source: Homilies on Second Corinthians (New Advent)