“[Fear] was an evil and corroding thread” (Page 67, Big Book)
Fear is primarily a spiritual question:
Will I trust God or trust myself?
Will I trust God to look after me, or will I trust in pathological, obsessive thinking, plans, schemes, designs, and plots?
Will I trust that the Twelve Steps are enough, or will I look for other methodologies, characterising them as ‘common sense’ or ‘worldly experience’?
Fear is secondarily a question of morals, of character:
Will I be brave or cowardly?
Will I adopt a constructive attitude or will I fret and complain?
Fear thus flows from the positioning of myself as my chief concern, rather than having the Doing of God’s Will as my chief concern.
Once admitted, it is then propagated by surrender to confounding, negative, unconstructive mental attitudes.
Once flourishing, it corrodes the nervous system, feeding off it like a parasite. Its action is then chiefly physical.
Self-regard distorts the mental, the psychological; the effect on the nervous system further affects the mental, the psychological too. The order: spiritual, then mental, then physical.
There is certainly a cognitive or reasoning element, as well, but this is secondary. Once I’ve let fear barge its way in, cognition and reason are co-opted to its cause. Attacking it with cognition and reason might clip its wings but won’t remove the problem at root.
This is why the problem must be solved at the level of the moral, through trusting God not self, having regard for the Doing of God’s Will not the arrangement of life to suit myself, and the development of courage in the place of cowardice.
The problem must also be solved at the level of the physical. When a fear thought tempts, it must be rejected. If admitted, its corrosive action will begin automatically.