The American Psychological Association:

Psychology is the study of the mind and behavior. The discipline embraces all aspects of the human experience — from the functions of the brain to the actions of nations, from child development to care for the aged. In every conceivable setting from scientific research centers to mental healthcare services, "the understanding of behavior" is the enterprise of psychologists

soul - noun

1. an entity which is regarded as being the immortal or spiritual part of the person and, though having no physical or material reality, is credited with the functions of thinking and willing, and hence determining all behavior Soul dictionary definition | soul defined

The definitions have been arbitrarily chosen. Others have considerable variance.

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