sonnetLIII

What is your substance, whereof are you made,
That millions of strange shadows on you tend?
Since every one hath, every one, one shade,
And you but one, can every shadow lend.
Describe Adonis, and the counterfeit
Is poorly imitated after you;
On Helen's cheek all art of beauty set,
And you in Grecian tires are painted new:
Speak of the spring, and foison of the year,
The one doth shadow of your beauty show,
The other as your bounty doth appear;
And you in every blessed shape we know.
   In all external grace you have some part,
   But you like none, none you, for constant heart.

The poet appears to be delving into the realms of Neo-Platonic metaphysiscs. The theory is that most of our experience is merely a shadow of reality. True or real existence is that of ideals, or ideal substances and forms. Every ideal or form has its shadow in the material world, and it is with the shadows that our senses have contact. All material things derive their shape and existence from these forms and therefore have something of the ideal in them, but it is only a severely restricted and cramped version of the ideal. Only the spiritual mind can grasp the true essence of things.

Yet here the beloved seems to be almost the universal ideal which gives form to all substance, since whatever lovely thing one might think of that appears in the world, he outdistances them all and gives them light and informs them with himself.

The poet therefore marvels at this fact, and sees within the beloved all the beauties of the old world inspired and given life, as it were, by him alone.The conclusion is somewhat at variance with some of the other critical sonnets, such as 33-5, 40-2 etc. It may be that the reference is to the enduring quality of the ideal Platonic form, which is essentially eternal and unchanging. Or it may be that all in the past is now forgiven and seen in a roseate light, the poet forcing this conclusion upon himself in deference to overpowering love, and as a means of overcoming pain. For it is the tradition of sonneteering that all cruelties by the beloved must be forgiven by the lover.

Further difficulties of interpretation are discussed in the notes below.

See also the additional notes to Thorpe's Dedication at the start of the Sonnets, and the Introductory Notes, which give an alternative interpretation of this sonnet.  

The 1609 Quarto Version

WHat is your ſubstance,whereof are you made,
That millions of ſtrange ſhaddowes on you tend?
Since euery one,hath euery one,one ſhade,
And you but one,can euery ſhaddow lend:
Deſcribe Adonis and the counterfet,
Is poorely immitated after you,
On Hellens cheeke all art of beautie ſet,
And you in Grecian tires are painted new:
Speake of the ſpring,and foyzon of the yeare,
The one doth ſhaddow of your beautie ſhow,
The other as your bountie doth appeare,
And you in euery bleſſed ſhape we know.
   In all externall grace you haue ſome part,
   But you like none,none you for conſtant heart.

Commentary

1. What is your substance, whereof are you made,
substance = essence, constituents, basic form. It corresponds in Neo-Platonic doctrine to the ideal or underlying form of things, from which individual instantiations arise in our world of sense data.
whereof = of what.
2. That millions of strange shadows on you tend?
strange = unknown, unconnected to you, unusual.
shadows
- in the Platonic sense of unreal things that derive their existence from forms and substances. Also in the conventional sense of shadows cast by objects. strange shadows could also be ghosts or spirits.
tend
- attend, as a servant attends a master, or a shepherd tends a flock OED(1) 3.b, 4.a. As in King Lear, where it is used in reference to Lear's former servants.
What need you five and twenty, ten, or five,
To follow in a house where twice so many
Have a command to tend you?
KL.II.4.260-2
However the meaning here is not obvious, and could include others, such as 'to have a disposition to attain to', OED (2) 2.a. Even possibly the geometric sense of 'subtend'.
3. Since every one hath, every one, one shade,
every one - everything, each substance, each separate entity. The antecedent cannot be shadows, because it is too awkward to talk of shadows having their own individual shade. The reference is therefore most probably to substance in line 1, or to things in general. Each perfect Platonic form has its own shadowy image in the world of men. However this does not make sense in terms of Platonism, because every ideal form can have any number of 'shades' derived from it. Thus the form of 'beauty' would transmit part of itself into the manifold instantiations of actual beauty in the world. Likewise for the form of 'roundness' or 'sphericity'. It is not true to say that 'sphericity' has only one 'shade' or example of itself in the physical world. The poet is perhaps hovering between the Platonic meanings and the idea that everyone only casts one shadow, or that everyone only has one spirit.
4. And you but one, can every shadow lend.
but one = being but one, being the unique essence of all.
can every shadow lend
= can give form and existence to every thing that is, can give a part of yourself, as a copy of the ideal. Again a somewhat difficult and quasi-philosophical idea which is based on Neo-Platonism. Beauty and goodness for example were ideas, forms, or ideals, which lent part of their substance to individual existences which possessed those qualities.
5. Describe Adonis, and the counterfeit
Describe Adonis - If one were to describe Adonis. The suggestion has greater immediacy because Shakespeare had in fact done just that, in his poem Venus and Adonis, which was dedicated to the Earl of Southampton.
the counterfeit
= the copy, the description, the painted version.
6. Is poorly imitated after you;
Is but a poor imitation of you.
7. On Helen's cheek all art of beauty set,
(If you were to) paint or describe Helen's face, using all the artifice at your command. Helen was the wife of Menelaus. She abandoned him and fled with Paris to Troy, thereby giving a motive for the start of the Trojan war. She was supposed to be the most beautiful woman in the world at the time, and in all recorded history. The story is mostly mythical and is linked in with the tale of the judgement of Paris. In return for selecting Aphrodite as the most beautiful of the three goddesses, (Athena and Hera being the other two), Paris was rewarded by having the world's most beautiful woman as his bedfellow (although she was already married). The subsequent enmity between the three Olympian goddesses helped to stoke the fires of vengeance in the Trojan war cycle. Many poets wrote subsequently of Helen with various gradations of praise or blame. Shakespeare portrayed Helen in Troilus and Cressida, written after the sonnets (but published in the same year).
8. And you in Grecian tires are painted new:
tires = attires, dress. The use of painted here and art in the line above perhaps suggest that some of the beauty was artificial.
9. Speak of the spring, and foison of the year,
foison = harvest, plenty, abundance. Compare
Earth's increase, foison plenty,
Barns and garners never empty.
Tem.IV.1.110-11.
10. The one doth shadow of your beauty show,
shadow - as above, lines 2 & 4. The Neo-Platonic doctrine is reasserted. The Spring is but the shadow (shade, instantiation) of your beauty.
11. The other as your bounty doth appear;
The foison of the year models its abundance on you.
12. And you in every blessed shape we know.

You are the universal perfection on which all subsequent copies are based.
blesséd shape - anything with qualities which we are inclined to praise; anything on which beauties of character appear to have been conferred. blesséd did not have the additional meaning it now has of a mild expletive, as in 'What the blessed point is there in writing all this?'

13. In all external grace you have some part,

The contrast is drawn between external grace, such as physical beauty, and unseen, internal perfection, in this case a constant heart.

14. But you like none, none you, for constant heart.

constant heart - apart from the obvious meaning of a true and loyal heart, it could refer to an unchanging substance or essence, re-inforcing the idea that the beloved was the basic form or ideal which infused itself into all other ephemeral forms of beauty in the sensate world. It is however slightly perturbing that the youth who has strayed and treated the poet badly, even to the extent of stealing his mistress, should here be endowed with a quality which he does not seem to possess.  The attribute of grace, which often is applied to a beloved, is also that of the 'onlie-begotten' of the gospels, and one suspects here a hidden religious meaning, since constancy and unchangeability were also attributes of the divine.