Sonnet CXIII

Since I left you, mine eye is in my mind;
And that which governs me to go about
Doth part his function and is partly blind,
Seems seeing, but effectually is out;
For it no form delivers to the heart
Of bird, of flower, or shape which it doth latch:
Of his quick objects hath the mind no part,
Nor his own vision holds what it doth catch;
For if it see the rud'st or gentlest sight,
The most sweet favour or deformed'st creature,
The mountain or the sea, the day or night,
The crow, or dove, it shapes them to your feature.
   Incapable of more, replete with you,
   My most true mind thus maketh mine eye untrue.

The 1609 Quarto Version

SInce I left you,mine eye is in my minde,
And that which gouernes me to goe about,
Doth part his function,and is partly blind,
Seemes ſeeing,but effectually is out:
For it no forme deliuers to the heart
Of bird,of flowre,or ſhape which it doth lack,
Of his quick obiects hath the minde no part,
Nor his owne viſion houlds what it doth catch:
For if it ſee the rud'ſt or gentleſt ſight,
The moſt ſweet-fauor or deformedſt creature,
The mountaine,or the ſea,the day,or night:
The Croe,or Doue,it ſhapes them to your feature.
   Incapable of more repleat,with you,
   My moſt true minde thus maketh mine vntrue.

Commentary

1. Since I left you, mine eye is in my mind;

Since I left you - the period of absence and its reason is not known. It may relate to the absences mentioned in 50-1 and 97-8, or to some other period of separation. Or it may be purely a distance caused by the poet 'ranging' to other loves, as described in 109 and 110.
mine eye is in my mind = my eye no longer is a seeing organ, but has become an adjunct to my mind and sees only what the mind wishes to see (as the following lines explain).

2. And that which governs me to go about

'That which directs me where to go, i.e., my eye'.
govern = steer, manage, direct.
to go about = to walk, to go on my way, to go about my business. In short, to participate in all the activities which rely on the use of the eye.

3. Doth part his function and is partly blind,

Doth part his function = departs from its usual activity and mode of operation.
his = its, and refers to mine eye or that which governs me to go about. This and all the remaining lines expand on how the eye parts his function and how it appears not to be seeing that which it does see.
is partly blind - presumably not totally blind, because it does see other objects, but the mind blots them out.

4. Seems seeing, but effectually is out;

Seems seeing = seems to be a seeing organ.
effectually = effectively, in effect, in practice.
is out = i.e. out of its socket, not there, removed, displaced, put out. But also with the sense of being extinguished and giving no light.

5. For it no form delivers to the heart

it = the eye.
no form = no image, no shape. Perhaps with a slight suggestion of the Platonic forms or ideals, although they are not specially relevant here. The youth was the form or ideal of all things beautiful in the world.
delivers to the heart = presents to the mind. Heart and mind in this context are interchangeable, although 'heart' tends to be used more when emotions are involved. The eye was envisaged as conveying the images of things it saw to the mind. See sonnets 46 and 47.

6. Of bird, of flower, or shape which it doth latch:

or shape which it doth latch - the science of optics hardly existed at the time, and theories of sight were based on ancient ideas, according to which the eye sent out particles or a flux which effectively latched on to the object at which the eye was looking.
latch - an emendation of Q's lack accepted by most editors.

7. Of his quick objects hath the mind no part,

his = its = the eye's.
quick objects = shapes which the eye has encountered and latched on to; living images. Onions (object n. 2.) gives 'Presentation (of something) to the eye or the perception'.
hath the mind no part = the mind does not share (the quick objects, the images, which the eye sees).

8. Nor his own vision holds what it doth catch;

his - as above, the eye's. SB however notes that the antecedent of his here and in line 7 could also be 'my mind'. The similarity of function of mind and eye is stressed in Sonnets 24 and 47. Compare also
CASS. Tell me good Brutus, can you see your face?
BRU. No Cassius, for the eye sees not itself
But by reflection by some other things.
CASS. 'Tis just;
And it is very much lamented Brutus,
That you have no such mirrors as will turn
Your hidden worthiness into your eye,
That you might see your shadow. JC.I.2.51-8.
Although the thought in JC differs from that expressed in this sonnet, it is evident that Shakespeare was engrossed by the uncertainty of reflections and the strangeness of mirror images, which he has depicted here in part in the partial echoes and reversions of words. Thus seems seeing, part partly, true untrue are like odd reflections in mirrors, shadows which have been turned into the mind by the use of an inner mirror.
holds what it doth catch = retains the images of the things onto which it latches.

9. For if it see the rud'st or gentlest sight,

it = the eye. But see previous note.
rud'st = crudest, most uncultured, most vile.

10. The most sweet favour or deformed'st creature,

sweet favour = delightful face, pleasing feature and appearance. deformed'st = most deformed.
Note that some editors emend this to The most sweet-favour'd or deformed'st creature so as to preserve the parallellism between this line and rud'st or gentlest, day or night etc.

11. The mountain or the sea, the day or night,

12. The crow, or dove, it shapes them to your feature.

it shapes them to your feature = it (the eye) transforms the images it has latched on to into images which look like you (have your features). In these five lines (8-12) the eye seems to act independently of the mind, and decides to convert all its sightings into visions of the beloved. However it has been driven to do this by the mind's insistence, so it remains unclear whether the eye or the mind is the instigator of these deceptions and transmutations.

13. Incapable of more, replete with you,

Incapable of more = being unable to do anything other than think of you. The subject is my mind in the next line.
replete with you = being filled entirely with thoughts of you.

14. My most true mind thus maketh mine eye untrue.

My most true mind - i.e. true because it sees only you, and therefore only sees what is worthy of being seen. However it makes the eye untrue because the eye does recognise other objects in the world around, but is forced by the mind to convert them to the shape of the youth. It is therefore giving a false record of what it has actually seen. true - also = constant, reliable, faithful.
mine eye - an emendation of Q's mine. Other emendations suggested are m'eyne, or changing My to Thy at the start of the line.