Distractive thoughts are the promptings of the demons and precursors of the passions, just as such promptings and mental images are also the precursors of particular actions. There can be no action, either for good or evil, that is not initially provoked by the particular thought of that action; for thought is the impulse, non-visible in form, that provokes us to act at all, whatever the action may be.
~St Gregory of Sinai
Prayerful Life 