Sonnet XXXVIII

How can my muse want subject to invent,
While thou dost breathe, that pour'st into my verse
Thine own sweet argument, too excellent
For every vulgar paper to rehearse?
O! give thy self the thanks, if aught in me
Worthy perusal stand against thy sight;
For who's so dumb that cannot write to thee,
When thou thy self dost give invention light?
Be thou the tenth Muse, ten times more in worth
Than those old nine which rhymers invocate;
And he that calls on thee, let him bring forth
Eternal numbers to outlive long date.
   If my slight muse do please these curious days,
   The pain be mine, but thine shall be the praise.

The poet is perhaps responding to a complaint that his output seems to be failing, and that he has for a while produced nothing new. The sonnet ties in with several others which praise the youth as the source of all inspiration. 53,78, 83, 84, 98, 99, 100, 103 all work on this theme in various ways.

Set at this point in the sequence, between sonnets of separation and despair, this sonnet helps to reinvest the youth with the previous beauty and fascination which perhaps had been waning under the influence of his faults. We are reminded once again of his surpassing excellence, his being, his very self, his own sweet argument which gives inspiration to all writers, a more powerful draught of inspiration than that provided by the outworn and outmoded old Nine Muses whom poets so tediously invoke to give life to their songs.

It is possible that this is a side swipe at the dependency on classical forms and material which were the staple of so much English writing of the period. Shakespeare's sonnet sequence is noticeably free of classical references. Yet we know from his poems, Venus and Adonis, The Rape of Lucrece, and his plays featuring Greek and Roman subjects, that he was familiar enough with the classical world to dramatize and use it when he wished.

One effect of the absence of classical allusion in the sonnets is that it brings the speaker closer to the reader. There are curtains and barriers between the two, but they stem more from emotional and linguistic complexity than from book learning. The Petrarchan and Elizabethan tradition of sonnet writing prefers that the beloved is a chaste and unassailable fair one, a Diana, and that the lover is a mad Leander prepared to swim the Hellespont to reach her and fling himself at her feet. Yet she remains icy and detached in virginal purity, her beauty inflaming him to still more acts of folly.

For whatever reason Shakespeare did not subscribe to this tradition. Either he wished to parody it, or he found it too constricting and insufficient to portray the emotions which battered him. There are often direct non-parodic echoes to sonnets of other writers, as here (see notes), and these echoes show how deeply Shakespeare was immersed in the literary traditions of his day, picking elements from it that suited his purposes.

The 1609 Quarto Version

How can my Muſe want ſubiect to inuent
While thou doſt breath that poor'ſt into my verſe
Thine owne ſweet argument,to excellent,
For euery vulgar paper to rehearſe:
Oh giue thy ſelfe the thankes if ought in me,
Worthy peruſal ſtand againt thy ſight,
For who's ſo dumbe that cannot write to thee,
When thou thy ſelfe doſt giue inuention light?
Be thou the tenth Muſe,ten times more in worth
Then thoſe old nine which rimers inuocate,
And he that calls on thee,let him bring forth
Eternal numbers to out-liue long date.
  If my ſlight Muſe doe pleaſe theſe curious daies,
  The paine be mine,but thine ſhal be the praiſe.

Commentary

1. How can my muse want subject to invent,
my muse = my poetic gifts, my inspiration. The nine Muses were goddesses of poetry in ancient Greece, each one dedicated to a specific branch of the art. The term is often used to imply that each poet has a personal Muse who looks after him/her.
want subject to invent = lack material for writing, show barrenness of inspiration.
2. While thou dost breathe, that pour'st into my verse
While thou dost breathe = while you are alive. To breathe and to inspire have approximately the same meaning, the latter being a Latinate word. (See OED.1. for inspire). It is basedon the Latin inspirare meaning to breathe upon or into. Hence the youth, while breathing, also breathes inspiration into the poet.
3. Thine own sweet argument, too excellent
Thine own sweet argument = thyself. argument is equivalent to 'subject' or 'theme'. We could therefore paraphrase 2-3 as 'While you are alive, who pour yourself as subject matter into my my verse.
too excellent - the suggestion is that the youth is too superior, too lofty to be the general subject of common writing.
4. For every vulgar paper to rehearse?
vulgar paper = common and cheap piece of writing. vulgar is derived from the Latin vulgus - the masses, the multitude, the crowd. With a somewhat pejorative flavour, and it retained much of that meaning in Shakespeare's day. One is reminded of a modern description of one of the 'royals' by someone attached to the aristocracy who rather disliked her - 'Vulgar, vulgar, vulgar. A vulgarian.'
paper stands for the writing which is on it. Similar to modern usage, as when a speaker delivers 'a paper' at a conference.
rehearse = repeat, go over (as one would do at a rehearsal).
5. O! give thy self the thanks, if aught in me
aught = anything (written by me).
6. Worthy perusal stand against thy sight;
Worthy perusal = that is worth looking at, or reading.
stand against thy sight = is strong enough, or worthy enough to be looked at by you. The conflation of idioms such as 'stand up to', 'stand in front of' (i.e. obscure), 'stand against an opponent', helps to contrast the poet's unworthiness and the youth's excellence, at the same time preventing us from believing absolutely in either.
7. For who's so dumb that cannot write to thee,

This is perhaps an echo of No. 46 of the series of the 'Delia' sonnets, by Samuel Daniel, published in 1592.
But I must sing of thee and those faire eyes,
Autentique shall my verse in time to come,
When yet th'vnborne shall say, loe where she lyes,
Whose beautie made him speake that els was dombe.
See the full text of Delia

dumb - dual meaning, as in the modern sense of 'unable to speak', or 'thick, unintelligent'.

8. When thou thy self dost give invention light?
invention = creativity. As in line1 above.
to give light to = to inspire, to lead the way.
9. Be thou the tenth Muse, ten times more in worth
It is interesting to see that Shakespeare has contrived to have the tenth Muse on line 9, (as if the youth were equivalent to all the previous nine), and the old nine he relegates to line 10, where they are out of place and useless.
10. Than those old nine which rhymers invocate;
those old nine = the Nine Muses of old. See the note above to line 1.
invocate = call upon. It was traditional for poets to invoke the Muses at the start of the poem. E.g. Homer at the start of The Iliad - "Sing, Oh Muse, of the wrath of Achilles'. I suspect that rhymers has a pejorative flavour to it. As in Ant & Cleo:
Nay, 'tis most certain, Iras: saucy lictors
Will catch at us, like strumpets; and scald rhymers
Ballad us out o' tune. AC.V.2.213-5.
11. And he that calls on thee, let him bring forth
calls on - Shakespeare converts invocate to its Anglo-Saxon equivalent, as if suggesting that the ancient language of the Muses is no longer necessary.
bring forth = create, write. Poems were the poet's children, hence the metaphor of child birth.
12. Eternal numbers to outlive long date.
numbers = verses.
to outlive long date = to last forever. (Literally - to live longer than a far distant (long) date.)
13. If my slight muse do please these curious days,
slight muse - powers of invention which are of no great significance.
curious = inquisitive, finicky, strange. A slight awkwardness arises from the sudden diminution of the muse, which in line 9 was to be the tenth Muse to replace all the previous nine, but now has become a slight thing barely able to raise interest from the 'curious age'. The reason is probably the need felt by the poet to emphasise in the final couplet his paucity of worth in comparison with that of the beloved.
curious = finicky, enquiring too closely into things.
14. The pain be mine, but thine shall be the praise.

the pain - the pain and labour of poetic creation.
the praise = the praise that accrues because of any worth in my poetry.