P12: The Mind, as far as it can, strives to imagine those things that increase or aid the Body’s power of acting.

Dem.: So long as the human Body is affected with a mode that involves the nature of an external body, the human Mind will regard the same body as present (by IIP17) and consequently (by IIP7) so long as the human Mind regards some external body as present, i.e. (by IIP17 S), imagines it, the human Body is affected with a mode that involves the nature of that external body. Hence, so long as the Mind imagines those things that increase or aid our body’s power of acting, the Body is affected with modes that increase or aid its power of acting (see Post. 1), and consequently (by P11) the Mind’s power of thinking is increased or aided. Therefore (by P6 or P9), the Mind, as far as it can, strives to imagine those things, q.e.d.