[2] Hagen published his 1868 book Zur Theorie der Halluzination, to define them as "illusions or sensory errors".
[4] Karl Jaspers further developed Kandinsky's ideas, emphasizing the "inner subjective space" as the locus of these experiences, where vivid sensory images occurred spontaneously but were devoid of the external reality attributed to hallucinations.
[5] Similarly, Eugen Bleuler conceptualized pseudohallucinations as perceptions marked by full sensory clarity and internal localization, while retaining intact reality testing.
[5] A common theme in these early perspectives was the differentiation of pseudohallucinations from hallucinations based on their subjective, internal nature and absence of sensory realness.
For example, auditory pseudohallucinations may involve hearing voices that are perceived as alien or attributed to other beings, but are acknowledged by the individual as originating from within the mind rather than from an external source.