In Sonnet 79, the speaker addresses an unnamed subject who, by all accounts, is exceedingly beautiful:
Men call you fayre, and you doe credit it
There is no denying the very real and "fair" beauty of the subject, but they are so much more than simply beautiful:
but the trew fayre, that is the gentle wit,
and vertuous mind is much more praysd of me.
Instead of focusing on the subject's fair outer beauty, the speaker notes that their real "fairness" is their "gentle wit" and "virtuous mind." Their character and intelligence are of far greater beauty than their loveliness. This is especially important because of the speaker's reflections about the nature of outer beauty in the following lines:
For all the rest, how ever fayre it be,
shall turne to nought and loose that glorious hew
It is possible to retain one's wit and intelligence for decades and decades. However, youthful beauty peaks and fades, making it impossible to retain. It is fleeting, no matter how outwardly beautiful one is. The subject's beauty comes directly from heaven, as all true beauty does, from the author of perfection:
That is true beautie: that doth argue you
to be divine and borne of heavenly seed:
deriv'd from that fayre Spirit, from whom al true
and perfect beauty did at first proceed.
The theme, therefore, is that inner beauty has far greater significance than outward appearances and is a gift from a perfect Creator.