I think it's the idea that your subconscious motor systems are way more powerful than your cortex. If you smoke cigarettes for example, it isnt really "you" deciding to light up each time, though your mind does have some say of when it schedules the smoke break. But rather it's the interplay between mind and body that is guiding the behavior. Outside of substance use, you could think of personality traits as guided by a bodily response too. If you're more neurotic and nervous by nature, it isnt your conscious decision to have the physiological response that you have. Its built into your genetic structure. I think the idea that our personality is entirely in our head to being too generous to what little say we actually have over most of what happens to us.