I guess I know what you mean. We all have these two; the one who percieves, classifies and acts and the one who on a deeper level directs our actions and thinking more than we often would be willing to admit.
The part that creates the dreams when we sleep or half sleep.
(split personality was just a feeble joke, sorry).
Seeing it in French and isolated from the rest of the poet's letter made it maybe a bigger mystery than it is.

Speaking of driving forces, I did not mean to make this post as my early morning good sense told me there are lots of things to be done and yet I do. Your post touching the essense of the little phrase.