1) Dominant: Introverted Thinking (Ti) - For the ISTP, logical thinking allows them to bring structural cohesion into the subjective aspects of t heir worlds and helps them to be disciplined.
They enjoy engaging challenges and goals for its own sake, rather than solely for the sake of competition. Rather than the more intellectual orientation of the INTP, the ISTP tends to focus more on reaching a level of athletic fitness or perfecting a certain set of practical skills.
As introverted thinkers, ISTPs are good solvers of practical problems. They quickly acquire a sense of how things work, and are able to think of ways to fix them.
2) Auxilitary: Extraverted Sensing (Se) - According to this function, the ISTP is novelty-seeking and somewhat hedonistic with respect to physical pleasures. It is this trait that predisposes them to attraction to extreme sports, as well as the ability to be unusually skilled at observing and being aware of their surroundings. Unlike INTPs, the ISTP tends to prefer watching sports to discussing ideas.
3) Tertiary: Introverted Intuition (Ni) - It is this function which allows the ISTP to have some degree of interest in abstract concepts, in a manner similar to that of the INTP, whose dominant function is Introverted Intuition. This function in the ISTP, however, is more unconscious than in an Introverted Intuitive type like the INTP or the INTJ.
4) Inferior: Extraverted Feeling (Fe) - The ISTP tends to become borderline obsessive-compulsive about the area of interest in which he is interested. It is in these modes that Judging (rather than Perceiving) modes tend to predominate. Their goals are oftentimes larger than they expected, but this causes them to only dig in their heels further and push for the prize, even if it may entail alienating themselves from others in the process.
This is an expression of the tendency of the ISTP to be an adrenaline junky. However, this tendency may tend to unbalance them. The ISTP, like the INTP, can to be somewhat childlike when it comes to being susceptible to the influence of cheesy love songs. They are likeiwse susceptible to "F" types who have the ability of overriding their predisposition to logical thinking and causing them to become infatuated.
Like the INTP, furthermore, the ISTP may be out of touch with his emotions, and perplexed by their tendency to have a life of their own, unable to explain their origin, nature and purpose. Like the INTP, furthermore, the ISTP is good at detaching himself from his emotions, particularly in circumstances which might otherwise rattle other personality types.
The ISTP likes others to feel comfortable around them and to maintain basic social harmony. Like the INTP, the ISTP has a desire for public approval. They want to be seen as good at what matters. They also tend to avoid conflict when possible. They have a tendency comparable to that of INTPs, in which they become preoccupied with workaholism to the point of alienating others, only to then reach back out to others.
The ISTP may have trouble expressing his or her feelings or grievances and may instead act on them in ways that can be maladaptive, such as deliberately avoiding people. One of the biggest challenges for the ISTP is the ability to understand the difference between hard work and inbalanced workaholism. He also needs to be able to meaningfully communicate with close friends and loved ones about uncomfortable topics in order to improve relationships and avoid unnecessary conflicts.
The ISTP comprises about 5 percent of the human population, with males outnumbering females around 3 to 1. Like the INTP, their dominant function is Introverted Thinking (Ti). And like the INTP, they exhibit high levels of self-discipline, independence and focus when it comes to their area of interest. They may take advantage of these personality traits to maximize their performance in sports and problem-solving.
Due to their predominantly practical orientation, the fact that their dominant function is Introverted Thinking may go unnoticed by many people. They tend to come off to other people as genuine and friendly. While approachable and friendly, their friendships revolve less around shared conversation and more around shared activities and interests.
Despite their lack of talkativeness, however, their desire for reputation and status may cause them to be more sociable in public settings at work than home. This is because, although they may love their family, these members do not provide the same ego boost their friends do in the public arena.
The ISTP tends to be more attuned towards stylish dress, and physically, th ey tend to be muscular and tall. They tend to enjoy athletic and fitness-oriented activities, as well as high-quality food, clothing, technology, and vehicles, such as classy cars and motorcycles. They tend to have good eye-hand coordination and mechanical intelligence as well. Just as the INTP as a high level of mental energy, the ISTP tends to have high levels of phsical energy. Where the INTP likes to explore different thoughts and ideas, the ISTP enjoys exploring the concrete world.
The ISTP, because of his preference for physical and hands-on activities, the ISTP sometimes under-performrs in the academic arena. However, this does not at all mean that the ISTP is unintelligent.
They are simply less interested in such activities than INTPs, and more interested in physical and hands-on activities. This is because they are less likely to be interested in abstract concepts, which they may find draining and boring, and are more invigorated by physical activities.
Phase 1 - The Introverted Thinking (Ti) function, their dominant function, emerges at this point in the form of their tendency to channel their energy into videogames or sports.
During this stage of life, if they take up musical activities, it tends to be something requiring a great deal of physical activity, such as drumming. As opposed to the easy-going ESTP, the ISTP tends to be more serious and focused.
Phase 2 - At this point, they begin to develop their auxilitary function, Extraverted Sensing (Se). The ISTP may decide to expand his purview and explore a broader range of experiences than what he is accustomed to. They may also engage in their tertiary function, Introverted Intuition (Ni), which may allow them to explore more abstract and philosophical questions than what they're used to.
Drenth, Dr. A.J. (2014-01-08). The 16 Personality Types: Profiles, Theory, & Type Development (Kindle Locations 84-86). Inquire Books. Kindle Edition.