The "midst" of something in the Bible represents the thing that is most central and most important to the spiritual state being described, the motivation that drives everything else. In general this will be something we love or feel, because at the core of things we are what we love; our loves define us.

(References: Apocalypse Revealed 44, 90, 268; Arcana Coelestia 2252, 2940, 10153, 10365, 10557, 10635, 10641)

928. 'The imagination of man's heart is evil from his childhood' means that the will part of man's mind is totally evil. This is clear from what has just been stated. 'The imagination of the heart' has no other meaning. Man supposes that he has a will for good, but in this he is utterly mistaken. When he does good, he does not act from his own will but from the new will which is the Lord's, and thus from the Lord. Likewise when he thinks and utters what is true, he speaks from the new understanding deriving from that new will, and so again from the Lord. For one who has been regenerated is an entirely new person who has been formed by the Lord. This also is why he is said to have been created anew.