Sonnet XVII

Who will believe my verse in time to come,
If it were filled with your most high deserts?
Though yet heaven knows it is but as a tomb
Which hides your life, and shows not half your parts.
If I could write the beauty of your eyes,
And in fresh numbers number all your graces,
The age to come would say 'This poet lies;
Such heavenly touches ne'er touched earthly faces.'
So should my papers, yellowed with their age,
Be scorned, like old men of less truth than tongue,
And your true rights be termed a poet's rage
And stretched metre of an antique song:
   But were some child of yours alive that time,
   You should live twice, in it, and in my rhyme.

The 1609 Quarto Version

WHo will beleeue my verſe in time to come,
If it were fild with your moſt high deſerts?
Though yet heauen knowes it is but as a tombe
Which hides your life , and ſhewes not halfe your parts:
If I could write the beauty of your eyes,
And in freſh numbers number all your graces,
The age to come would ſay this Poet lies,
Such heauenly touches nere toucht earthly faces.
So ſhould my papers (yellowed with their age)
Be ſcorn d,like old men of leſſe truth then tongue,
And your true rights be termed a Poets rage,
And ſtretched miter of an Antique ſong.
  But were ſome childe of yours aliue that time,
  You ſhould liue twiſe in it,and in my rime.

Commentary

1. Who will believe my verse in time to come,

Who will believe = who (among the readers of the future) will believe

2. If it were filled with your most high deserts?

If it were = even if it were. The poet modestly implies that the deserts and superb qualitiesof the youth are too large and abundant for his pen to describe adequately. He wishes to fill his verse with them, but finds that it is beyond him. The clash of tenses between will l.1 and were l.2 has worried some commentators, but the meaning is clear enough.

3. Though yet heaven knows it is but as a tomb

but as a tomb = like a tomb. A hint of the exegi monumentum theme which has already been sounded in the previous two sonnets and reaches fruition in 63, 65 and especially 81:
Your monument shall be my gentle verse,
Which eyes not yet created shall o'er-read;
And tongues to be your being shall rehearse,
When all the breathers of this world are dead;
Here it is the negative side of tombs which is emphasised. They hide life, and do not disclose it.

4. Which hides your life, and shows not half your parts.

parts = qualities, talents, characteristics. But also with a hint of bodily parts.

5. If I could write the beauty of your eyes,

If I could write = if I could describe

6. And in fresh numbers number all your graces,

in fresh numbers number = in fresh verses enumerate; the first numbers is a noun meaning verses, the second is a verb meaning to count. Verses were sometimes referred to as numbers because of their musical quality, and the fact that one could count the number of stresses to a line etc.
graces = gracious qualities.

7. The age to come would say 'This poet lies;

The age to come = people who live in future ages.

8. Such heavenly touches ne'er touched earthly faces.'

touches = descriptions, strokes of a painter's brush (figuratively);
ne'er touched = never belonged to, never were placed on, were never relevant to.

9. So should my papers, yellowed with their age,

my papers - the papers on which my sonnets are written; the sonnets themselves.
yellowed with their age - white paper discolours as it ages. There is probably a hint also of the yellowing of skin with age, as in old men, who figure in the next line.

10. Be scorned, like old men of less truth than tongue,

Old men were proverbially thought to gabble endless nonsense (tongue = speech). Justice Shallow depicts the type in 2H.4.III.2.

11. And your true rights be termed a poet's rage

true rights = the rights of praise which are your due because of your beauty;
a poet's rage = the frenzied inspiration which drives a poet to create. In the ancient world there was not a great distinction made between a poet and a seer, the latter especially being thought to be inspired with divine fervour. Cassandra is probably the original, the prophetess seized by the inspiration of Apollo, but doomed never to be believed. At Delos the priestesses were thought to have inhaled sulphurous fumes which intoxicated them, and in such a state they uttered their prophecies. In Virgil Aeneas visits the Sybil in her cave on the coast of Euboean Cumae. 'Meanwhile the prophetess, who had not yet submitted to Apollo, ran furious riot in the cave, as if in hope of casting the God's power from her brain. Yet all the more did he torment her frantic countenance, overmastering her wild thoughts, and crushed her and shaped her to his will. So at last, of their own accord, the hundred tremendous orifices in the shrine swung open, and they carried through the air the answer which the prophetess gave.' (Aeneid Bk VI, Penguin translation). Apollo was the god of prophecy, but also, with his lyre, the god of poetry. For poetry sprang originally from a religious tradition. The ancient traditions, through the learning of poets such as Spenser, Sydney, Drayton and Jonson, had permeated through to the consciousness of the age, and the poet's frenzy became a byword for poetic creation.

12. And stretched metre of an antique song:

This was one of Keat's favourite lines. stretched metre suggests that the metre of the line in old poems was irregular, or perhaps too long. antique as well as meaning old, could have a secondary meaning of bizarre, odd, slightly insane.

13. But were some child of yours alive that time,

But were some child = but if some child were.
that time = at that time in the future when these verses are perused (and doubted).

14. You should live twice, in it, and in my rhyme.

This is the final encouragement to the youth to have children, and it is set alongside his potential immortality through the poet's verse, as perhaps the better of the two alternatives.