Stress. Hostile conditions. Just enough so that the autonomic nervous system (responsible for the "Fight or Flight" response) isn't constantly hyperactive to the point where it wreaks hormonal havoc, nor to the point where it gives up (the freeze response), and yet not so below the level of awareness as to inspire complacency or inertia.
Life is movement, it is flow; what remains still and cannot adequately react to pertinent stimuli is, for all intents and purposes, dead.
Originally published on my Facebook page December 13th, 2018.