H ow do external observers investigate someone else’s psyche systematically if they cannot apprehend it with their senses?

Behaviourism, a movement in psychology, maintains we cannot study the psyche at all because its immateriality renders it inaccessible to measurement. So behaviourists, using cause-effect methodology, measure only the directly observable: the environment and behaviour.

For example, once behaviourists know that a child likes Smarties (environment), they can get the child to do anything they want (behaviour) by giving and withholding (manipulating) a measurable number of environmental Smarties.

This example is a simplification of course, but with the eventual isolation of all the causes of behaviour, and all rewards like Smarties, behaviourists have faith that behaviour might eventually be completely predicted and controlled; hence, psychology is sometimes defined as the scientific study of the behaviour of humans and animals.

Many psychologists object to this exclusively behavioural definition saying that a complete denial of the psyche prevents them from making inferences about the phenomena behind behaviour.

If psychologists can explain behaviour by referring to consciousness, cognition, thought or emotion, then they can risk a much richer range of predictions about behaviour.

Thus many psychologists regularly construct theories about the psyche, but they may still choose to base them on the experimental observation of behaviour played out in measurable environmental circumstances.

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