Introspection is a conscious mental and usually purposive process relaying on thinking/reasoning/examination of one's own thoughts and feelings. It can also be called contemplation on one's self, as opposed to extrospection, the observation of things external to one's self. Introspection may be used synonymously with self-reflection and used in a similar way. Introspection is like the activity described by Plato when he asked, "...why should we not calmly and patiently review our own thoughts, and thoroughly examine and see what these appearances in us really are?" (Theaetetus, 155)
Cognitive psychology accepts the use of the scientific method, but rejects introspection as a valid method of investigation. It should be noted that Herbert Simon and Allen Newell identified the 'thinking-aloud' protocol, in which investigators view a subject engaged in introspection, and who speaks his thoughts aloud, thus allowing study of his introspection.
On the other hand, introspection can be considered a valid tool for the development of scientific hypotheses and theoretical models, in particular, in cognitive sciences and engineering. In practice, functional (goal-oriented) computational modeling and computer simulation design of meta-reasoning and meta-cognition are closely connected with the introspective experiences of researchers and engineers.
Introspection was once an acceptable means of gaining insight into psychological phenomena. Introspection was used by German physiologist Wilhelm Wundt in the experimental psychology laboratory he had founded in Leipzig in 1879. Wundt believed that by using introspection in his experiments he would gather information into how the subjects' minds were working, thus he wanted to examine the mind into its basic elements. Wundt did not invent this way of looking into an individual's mind through their experiences; rather, it can date to Socrates. Wundt's distinctive contribution was to take this method into the experimental arena and thus into the newly formed field of psychology.
Unlike oxygen, honey bees and Mustang convertibles, in humans there is a considerable amount of individualization, no doubt arising from nature's increased reliance on individual imagination and judgment ("reasoning"). But since this is an order problem rather than a privacy problem the solution is, not to banish introspection, but rather to differentiate (stratify) between the more evolved individualized features and the more mechanical, isomorphic processes lower in the evolutionary scheme of things. Once accomplished, the individualization can then be dealt with by applying corresponding amounts of abstraction and generalization to those features where individualization appears to be most rampant (Rehabilitating Introspection).