The 'mentalistic' theory, which is by far the older, supposes that the variability of human conduct is due to the interference of some non-physical factor, a 'spirit' or 'will' or 'mind' (Greek 'psyche', hence the term 'psychology') that is present in every human being. This spirit, according to the mentalistic view, is entirely different from material things and accordingly follows some other kind of causation or perhaps none at all.
- Bloomfield (1935), Pag. 32