What does "Phantasy" mean?
For the Individual, "Phantasy" is, in origin, a psychoanalytic term; which, in general - is taken to mean the unconscious imaginings or ideas related to the unfulfillment of previously expressed needs (originally in unconscious ways as a child - though certainly this process manifests throughout life) projecting an amalgamation of hopes, dreams, and wishes which manifest as an immature (i.e. "underdeveloped", due to a lack, or absence, of experience) "hypothesis" - based on what has been introjected and internalized from familial, societal, and cultural conditioning - as to what the potential fulfillment of those needs could be via said conditioning.
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As such, the "Problem" of Phantasy can be seen via the definition offered above in that much of (though, to be clear, not all of) what we hope, dream, and imagine "fulfillment" - and thus "wholeness" - to be is based on what has been introjected and/or internalized via systemic conditioning. In other words, our hypothesis - or as Adler calls it, our "Life-Plan" - is built on artificial constructs of "reality".
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Collectively, or in spiritually esoteric terms, we can call the total of all Phantasy to be "Maya" (Vedanta) or "Demiurge" (Platonism), "Yaldabaoth" or "The Evil" (Gnosticism), and so on.