sonnetLXV

Since brass, nor stone, nor earth, nor boundless sea,
But sad mortality o'ersways their power,
How with this rage shall beauty hold a plea,
Whose action is no stronger than a flower?
O! how shall summer's honey breath hold out,
Against the wrackful siege of battering days,
When rocks impregnable are not so stout,
Nor gates of steel so strong but Time decays?
O fearful meditation! where, alack,
Shall Time's best jewel from Time's chest lie hid?
Or what strong hand can hold his swift foot back?
Or who his spoil of beauty can forbid?
   O! none, unless this miracle have might,
   That in black ink my love may still shine bright.

The theme of mortality is continued, with the same items of earthly longevity and stability quoted as in the preceding sonnet - brass, stone (towers), earth, and the all hungry but mortal ocean. Their stability is mere sham and their strength a complete illusion. How, in all this wasteful ruin, may such a fragile thing as beauty survive? It seems indeed to be an impossibility. Yet there is one hope, a slender chance, that the immortality of verse will hold out against the advancing destruction of time and that the youth's ever young beauty will be preserved, by some miracle, in the mortal words that the poet writes.

  Although this is a recurrent theme in the sonnets, as for example in 18, where we are assured that:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee

here the sense of threatened personal loss and the invincibility and inevitability of time's advancing destruction make the remedy more poignant and less assured. Perhaps words also will be swept aside in the general wrack. Perhaps the ink will not be strong enough to outshine death, and its blackness will turn out to be symbolic of darkness to come.
Whatever the outcome, we are left with a sense of sorrow and the awfulness of impending doom for the youth and for all humanity.

The 1609 Quarto Version

SInce braſſe,nor ſtone,nor earth,nor boundleſſe                                                                                           ſea,
But ſad mortallity ore-ſwaies their power,
How with this rage ſhall beautie hold a plea,
Whoſe action is no ſtronger then a flower?
O how ſhall ſummers hunny breath hold out,
Againſt the wrackfull ſiedge of battring dayes,
When rocks impregnable are not ſo ſtoute ,
Nor gates of ſteele ſo ſtrong but time decayes?
O fearfull meditation , where alack,
Shall times beſt Iewell from times cheſt lie hid?
Or what ſtrong hand can hold his ſwift foote back,
Or who his ſpoile or beautie can forbid ?
   O none,vnleſſe this miracle haue might,
   That in black inck my loue may ſtill ſhine bright.

Commentary

1. Since brass, nor stone, nor earth, nor boundless sea,
brass, stone, are the paradigms of long lasting substances. earth and boundless sea are also long lasting, and superior in that they are of near boundless extent. These are all things which ought by their nature to be capable of holding out against mortality.
2. But sad mortality o'ersways their power,
sad mortality = mortality which causes sadness; solemn, ugly, hideous mortality.
o'ersways their power = has greater power than they have. 'To exercise sway over' is to rule over. The term is not much used nowadays in this sense (OED.n.5.) but is found in such phrases as 'to hold sway over'. to oversway is to be superior to one who already holds sway.
3. How with this rage shall beauty hold a plea,

rage is used in two previous sonnets in a similar context, to exemplify the blind fury of Time's destructiveness.
And barren rage of death's eternal cold?
13.
And brass eternal slave to mortal rage;
64.
It is suggestive of the madness of an unreasoning tyrant, or the irrationality of someone who has gone berserk.
hold a plea
- hear a plea, as in a court of law, where an action might be advanced for a stay of execution. SB thinks it is a misapplied term, the precise meaning being '"to try an action" - i.e. to have jurisdiction, to be judge' (SB p.246.n.3.) OED 1.b. does indeed give the definition 'to try an action' with various examples, e.g.
1570–6 Lambarde Peramb. Kent (1826) 182 Having a court...in which they hold plea of all causes and actions, reall and personall, civill and criminall.
But one suspects that the meaning is the more general one of sustaining or defending a plea, which the average layman might take it to be. OED also gives under "hold" (3.d.) the meaning: "To sustain, bear, endure, ‘stand’ (some treatment)". with the following examples:
1606 W. Crawshaw Romish Forgeries Aija, If the matter will not hold plea, and if my proofe be not substantiall. 1607 Shakes. Cor. iii. ii. 80 Now humble as the ripest Mulberry, That will not hold the handling.
The fact that the first example contains the phrase 'hold plea' works in favour of taking a more general sense of the phrase, rather than the restricted one given as OED.1.b. The imagery is that of a timorous subject defending an action before an enraged and absolute judge who is clearly not going to take any notice of the plea offered.

4. Whose action is no stronger than a flower?
action - the legal terminology continues. The legal action undertaken by beauty to prevent destruction is no more effective than a flower attempting to stop the march of time. The metaphor ranges beyond the merely legalistic, and sets up the image of the flower being trampled by the boot of the tyrant.
5. O! how shall summer's honey breath hold out,
summer's honey breath = the balmy, perfumed breezes of summer, the scent of flowers.
hold out - an echo of hold a plea above.
6. Against the wrackful siege of battering days,
wrackful - bringing devastation, wreckage and ruin. Full of such disasters. Based on the word wrack, meaning ruin and devastation (OED.n.1.2.a.) An alternative spelling perhaps to wreckful (although OED does not give it as such).
the wrackful siege of battering days - the image is of siege warfare, and the battering ram, which was a large beam of wood swung with great violence against the gates of a city to batter them down . The end of a successful siege (from the attackers' point of view) was the capture and destruction of the city.
7. When rocks impregnable are not so stout,
rocks impregnable - i.e. they are impregnable to any human agency, but time can overpower them. impregnable - unassailable. A word often applied to fortresses and other strong military defence points.
8. Nor gates of steel so strong but Time decays?
gates of steel - the defence of a walled city. Shakespeare describes Troy's gates in Troilus and Cressida:
.........Priam's six-gated city,
Dardan, and Tymbria, Helias, Chetas, Troien,
And Antenorides, with massy staples
And corresponsive and fulfilling bolts,
Sperr up the sons of Troy.
TC.Pr.15-17.
They protect the inhabitants for the interim but are no defence against the ravages of time.
but Time decays - but even them Time causes to decay. Decay is not normally a transitive verb, and here it is left uncertain as to how Time achieves its end of universal decay.
9. O fearful meditation! where, alack,
fearful - to be feared, causing fear. The fearful meditation is that which has already been stated, and the fears which are about to be stated.
10. Shall Time's best jewel from Time's chest lie hid?
Time's best jewel - the most precious thing in the world; the beloved youth.
Time's chest = the treasure chest in which Time stores all the things it steals. A coffin.
11. Or what strong hand can hold his swift foot back?
The hand/foot imagery suggests the possibility of a) tripping up Time as it speeds on its way; b) the helplessness of a hand raised in a useless and abandoned attempt to stop a far stronger and swifter adversary.
12. Or who his spoil of beauty can forbid?
spoil = spoliation, despoilment, disfigurement. SB defends the Q reading of who his spoil or beauty can forbid. He takes it to mean 'Who can deny Time the enjoyment of his loot (spoil) and who can forbid the youth to be beautiful?' (SB.p.247.n.12).
13. O! none, unless this miracle have might,
O none - this is the answer to the two questions posed in lines 11-12. No answer is given to the first question of lines 9-10, where ... shall time's best jewel from time's chest lie hid? But in a sense all three questions are answered, if we allow the miracle that the jewel may be hidden in the lines of this (and other) sonnets, that the poet will hold back the swift foot of time, and that the despoliation of beauty will be made good by the descriptions of his beauty to be found in these verses.
14. That in black ink my love may still shine bright.

my love = you, the beloved youth; my love for you. The blackness of the ink opposed to the shining brightness of the youth described in the sonnets is part of the miracle of his preservation.