(Greek: psyche, soul; logos, doctrine)
In the most general sense the science of the soul and its operations.
The ancients, Aristotle and the medieval Scholastics, treated the whole matter together; but in modern times, owing to the development of experimental seiences, a distinction is made between empiric psychology which, by use of experimental method (e,g., memory tests), studies data of consciousness, their laws, and correlation of body and soul; and rational (philosophical) psychology which, by argumentation, principally deductive, studies the causes and principles of psychic activity on the basis of experimentally established facts (e.g., from operations of intellect and will argues the spirituality of human soul).
To be complete, both empiric and rational psychology should embrace study of soul and its manifestations wherever found, in plants, brute animals, men.
Since the goal of rational psychology is the establishment of the ultimate nature of soul and its manifestations, it must begin with data (normal and abnormal) supplied either by experiences of every-day life or by scientifically controlled experiment.
In both cases the method to be followed is that of introspective observation aided and corrected by other supplementary sources, and of careful analysis of the various activities of living things.
Based on the results of such study, rational psychology proceeds to an explanatory account of the nature of the agent or subject of these activities (soul), with its chief properties, e.g., the nature of life and of soul, the different kinds of life (vegetative, sensitive, intellectual), knowledge and appetition, spirituality and immortality of the human soul, and quite generally the question of the origin of life and of species (Evolution or Transformism).
New Catholic Dictionary