Active imagination was developed by Carl Jung between 1913 and 1916, during what he called his own "confrontation with the unconscious" — a period of deliberate, terrifying engagement with his own depths that produced the material later compiled as The Red Book. He did not theorise the method first and then apply it; he discovered it by doing it, then spent decades understanding what had happened.
The core principle is simple and radical: the unconscious contains autonomous figures — the Shadow, the Anima/Animus, the Wise Old Man, the inner critic, the wounded child — that exist as independent psychic entities with their own perspectives, intentions and demands. Active imagination treats these figures as real — not as projections to be analysed away, but as genuine inner presences to be encountered, dialogued with, and related to.
This is not the same as believing they have literal, metaphysical existence outside the psyche. It is recognising that, for practical purposes of psychological growth, they function as if they do — they surprise you, say things you did not expect, resist your interpretations, and have effects in your life whether or not you acknowledge them. Engaging them consciously is enormously more effective than trying to manage their effects from outside.
Active imagination occupies a unique position between dream and waking: it is more conscious than dreaming (you are fully awake and present) but more receptive than ordinary thinking (you do not control or direct what the unconscious offers). The technical challenge is holding this middle position — ego fully present but not dominating, unconscious speaking but not overwhelming.