Reality testing occupies a peculiar and contested position within the depth-psychology corpus: it is simultaneously a clinical procedure, an ego function, a neurological capacity, and a philosophical problem. Freud introduced the concept structurally, locating it in the ego’s capacity to distinguish internal wish-fulfilment from external perception — the failure of which underlies the hallucinatory wish-psychoses he described in *The Interpretation of Dreams*. In the clinical literature, Worden and Yalom treat reality testing as an active therapeutic technique: the counsellor or the group confronts distorted cognitions, tests irrational guilt against evidence, or deputises group members themselves as agents of correction when individual transference has rendered the therapist’s own voice untrustworthy. Hillman mounts a sharp archetypal challenge to this entire project, arguing that reducing psychic reality to outer, objective verifiability — the Enlightenment legacy of Esquirol — impoverishes the soul’s own epistemological domain. McGilchrist re-frames the question neurobiologically: reality testing is predominantly a right-hemisphere function, localisable to the right dorsolateral prefrontal cortex, and its failure — manifest as delusion, incorrigibility, and unwarranted certainty — is the signature pathology of a left hemisphere unchecked by its counterpart. The resulting tension — between reality testing as rational corrective, as culturally imposed epistemology, and as hemispheric biology — marks the term as one of the richest and most contested nodes in the library.