the act of imagining

" The imaginatio, or the act of imagining, was thus a physical activity that could be fitted into the cycle of material changes, that brought these about and was brought about them in turn. In this way the alchemist related himself not only to the unconscious but directly to the very substance which he hoped to transform through the power of the imagination. The singular expression " astrum" ( star ) is a Paracelsan term, which in this context means something like " quintessence ". Imagination is therefore a concentrated extract of the like forces, both physical and psychic. "

C.G. Jung
Psychology and Alchemy
page 278