Sir Isaac Newton is best approximated by the four-letter MBTI equivalent INTP when his psychology is examined through the framework of Chapter X of Carl Jung's Psychological Types. This conclusion should be treated as a modern translation of Jung's typology rather than as a literal MBTI diagnosis, because Jung himself did not use the later MBTI four-letter system. The strongest basis for the typing is the combination of introverted thinking as the dominant function, supported by intuition, with a comparatively less developed relationship to feeling and sensation. The central question is therefore whether Newton's consciousness was primarily oriented by the subjective intellectual principle or by external objects. Chapter X makes this distinction crucial. Jung states that introverted thinking is oriented principally by the subjective factor. It may deal extensively with concrete reality, but its decisive movement is inward, toward the development of an internally coherent idea rather than toward the object for its own sake. Facts serve the construction and testing of an intellectual conception rather than constituting the ultimate purpose of thought. This description fits Newton particularly well. His importance was not simply that he collected observations or performed calculations. His distinctive achievement was the construction of abstract explanatory principles capable of organizing apparently separate phenomena. The formulation of universal laws from diverse observations reflects the introverted thinker's tendency to seek an underlying conceptual structure. In Jung's terms, this is closer to thinking that begins from an intellectual conception and seeks to determine how external facts can be understood through it than to thinking whose primary orientation is the accumulation and classification of external data. The distinction from extraverted thinking is particularly important. Jung describes extraverted thinking as being determined primarily by objective data and oriented toward external facts or generally accepted ideas. It is productive when it discovers new facts or creates general conceptions from empirical material. Newton certainly possessed extraordinary empirical and mathematical competence, but the deeper character of his intellectual work suggests that empirical material was incorporated into a more internally organized theoretical system. His mathematical and physical investigations were not merely descriptive. They attempted to penetrate the principles behind observable phenomena. The second letter, N, is therefore most plausibly represented by intuition. Jung's auxiliary-function discussion explains that a conscious primary function can be accompanied by an auxiliary function of a fundamentally different nature. In the case of a rational thinking type, the auxiliary function would belong to the irrational functions, particularly sensation or intuition. Newton's extraordinary tendency toward abstraction, theoretical possibility, hidden principles, and the search for relationships beneath observable events makes intuition the more plausible auxiliary function than sensation. His thinking was not confined to what was immediately observable. It continually sought what the observed world implied beyond itself. The P designation in the MBTI translation requires some qualification. In conventional MBTI terminology, INTP is a perceiving type, but Jung's original system does not map perfectly onto this letter. The apparent contradiction disappears when the four-letter label is treated as an approximate modern equivalent rather than a direct transcription. Jung's actual distinction concerns the dominance of a rational function, in this case thinking, together with the direction of psychic orientation. The important point is that Newton's dominant process is introverted thinking, not that he literally possessed an MBTI-style preference measured by a modern questionnaire. Newton's introversion also fits the broader description of Jung's introverted attitude. The introvert's relationship to the object is comparatively negative or secondary, while psychic energy is concentrated inward. Jung emphasizes that the introverted thinker can develop ideas with considerable independence from immediate external demands and may show little concern for making those ideas socially attractive or easily accessible. Newton's reputation for intense intellectual concentration and relative detachment from ordinary social concerns is therefore compatible with this pattern, although biographical evidence alone should never be treated as sufficient proof of type. His likely inferior functions can also be understood through Jung's model. The dominant function tends to be highly differentiated and conscious, while less differentiated functions remain comparatively unconscious and can emerge in more primitive or spontaneous forms. For an introverted thinking type, this suggests that personal feeling and direct engagement with immediate concrete life may be less consistently developed than intellectual analysis. This does not mean Newton lacked emotion, values, or sensory awareness. Rather, those functions would not be expected to organize his conscious personality to the same degree as thinking. Overall, INTP is the strongest four-letter MBTI equivalent for Newton when Chapter X is used as the theoretical foundation. The decisive feature is not simply that Newton was intelligent, scientific, or solitary. It is that his intellectual activity appears best characterized by an inwardly directed search for coherent principles, with empirical reality serving as material for a deeper conceptual reconstruction. That corresponds closely to Jung's introverted thinking type, while his abstract and possibility-oriented approach provides a reasonable basis for identifying intuition as the auxiliary function. The resulting INTP label is therefore best understood as a modern shorthand for a Jungian structure centered on introverted thinking and supported by intuition, rather than as a claim that Newton himself would necessarily have received the same result on a contemporary MBTI assessment.