No Rack can torture me —
My Soul — at Liberty —
Behind this mortal Bone
There knits a bolder One —

You cannot prick with Saw —
Nor pierce with Cimitar —
Two Bodies — therefore be —
Bind One — The Other fly —

The Eagle of his Nest
No easier divest —
And gain the Sky
Than mayest Thou —

Except Thyself may be
Thine Enemy —
Captivity is Consciousness —
So's Liberty –
Fr649 (1863)  J384

I've been struggling with this poem for a few weeks now. Oh, it goes along tamely enough for the first three stanzas. Dickinson builds the unremarkable case that the soul is not constrained by the "mortal Bone," but is instead, like the eagle, a creature of flight and freedom. It can "divest" itself of the body and "gain the Sky." It is the "bolder" of the two Bodies – and no wonder! Unlike your flesh body, it cannot be hurt by saws, scimitars, or even torturers' tools. It cannot even be bound; it can simply fly away.

But then there is the ambiguity of the fourth stanza: "Except Thyself may be / Thine Enemy –". Which phrases are these two lines are attached to? Is Dickinson saying that the soul can gain the sky unless you are your own enemy Or is she saying that unless you are your own enemy, Captivity and Liberty are both synonymous in some way with Consciousness? If forced to choose, I would opt for the latter reading although I don't think it is a significant point.

In terms of liberty enabling a consciousness-enhancing captivity, Adrienne Rich, in her marvelous essay on Dickinson, "Vesuvius at Home," recounts the following:

[Dickinson's] niece Martha told of visiting her in her corner bedroom … and of how Emily Dickinson made as if to lock the door with an imaginary key, turned and said, “Matty: here’s freedom.”

The poet's locked room becomes a metaphor for the mind, the enclosed space figures as the skull, the poet as the soul. Freed from outside care by the confinement, the soul may boldly venture beyond earthly realms and quotidian concerns. Consciousness emerges from the captivity; Consciousness whose liberty gains the Sky – and Dickinson often uses 'Sky' in place of 'heaven', 'cosmos', and even 'God' (ED Lexicon). 
        An interesting insight comes from Boston University's Thomas Finan who in 2015 wrote "'Captivity is Consciousness': Consciousness and its Revisions in Dickinson’s Poetry," The Emily Dickinson Journal, vol. 24 no. 2, pp. 24-45. Finan refers to the mid-1800s and the "major philosophical and literary themes involved in the rise of 'consciousness' [as] a concern with the way in which consciousness could imprison through its mediating and unifying capabilities." Finan is convinced that despite this concern, Dickinson found that "the walls of consciousness" could provide a barrier "behind which the self can withdraw." The confining barriers "can provide the prospect of liberty." Yes, much like turning the key in the lock of the door.
But besides all of that, the ambiguously anchored phrase, "Except Thyself may be / Thine Enemy –", remains of interest. Beyond the notion that we can defeat ourselves, there is Lucifer's definitive exclamation in Paradise Lost: "The mind is its own place, and in itself can make a heaven of hell, a hell of heaven."(John Milton, Paradise Lost, Book I, lines 254-5). Surely, the mind that turns a heaven into a hell is its own enemy; the one that does the reverse achieves the Sky.

Dickinson has written about this self-enmity before:
    • in "One need not be a Chamber – to be Haunted – " (J670 / Fr407), it is "Ourself behind ourself" that should scare us more than any ghost;
    • in "They shut me up in Prose" (J613 / Fr445), it is as laughable to put a bird behind a fence as to command a poet to stick to prose or a child to stay quietly in her room;
    •  in "A Prison gets to be a friend – " (J652 / Fr456), the 'Geometric Joy" of prison is of our own making and Liberty avoided "like a Dream."

But perhaps her most in-depth examination is yet to come. In J642/Fr710, Dickinson asks, "But since Myself – assault Me – / How have I peace / Except by subjugating / Consciousness? This seems contrary to the current poem where Consciousness is the desirable face of both captivity and liberty. Here, Dickinson wants peace at the expense of Consciousness yet cannot imagine how to abdicate herself of herself. The question is almost one of transcendental meditation. The whole poem is worth reading here:

Me from Myself — to banish —
Had I Art —
Impregnable my Fortress
Unto All Heart —

But since Myself — assault Me —
How have I peace
Except by subjugating
Consciousness?

And since We're mutual Monarch
How this be
Except by Abdication —
Me — of Me?
                             J642,  Fr710  (1863)