From Wikipedia

Introverted thinking is the thinking function that is subjective (being introverted). The nature of introverted thinking means that it is primarily concerned with its “subjective idea” and insights gained by formulation over facts and objective data. Whereas Extraverted Thinking is most like Empiricism, Introverted Thinking is most similar to Rationalism.

“Just as Darwin might possibly represent the normal extraverted thinking type, so we might point to Kant as a counter-example of the normal introverted thinking type. The former speaks with facts; the latter appeals to the subjective factor. Darwin ranges over the wide fields of objective facts, while Kant restricts himself to a critique of knowledge in general. But suppose a Cuvier be contrasted with a Nietzsche: the antithesis becomes even sharper.”

“The introverted thinking type is characterized by a priority of the thinking I have just described. Like his [p. 485] extraverted parallel, he is decisively influenced by ideas; these, however, have their origin, not in the objective data but in the subjective foundation. Like the extravert, he too will follow his ideas, but in the reverse direction: inwardly not outwardly. Intensity is his aim, not extensity. In these fundamental characters he differs markedly, indeed quite unmistakably from his extraverted parallel. Like every introverted type, he is almost completely lacking in that which distinguishes his counter type, namely, the intensive relatedness to the object.”