ISTP, or Introverted Sensing Thinking Perceiver, is a label borrowed from MBTI nomenclature and now applied to the Jungian Cognitive Function set {Ti, Se, Ni, Fe}.

Dominant: Introverted Thinking (Ti)

"Above all, I like to maintain a consistent outlook that fits well with the things I like to do and the way I like to do them. I'll stay out of your way and, as long as I offer you that courtesy, I expect the same in return. I like to work with things where I can figure out the best approach myself and then tweak and adjust the pieces and see or hear or smell an immediate change or improvement. If something isn't doing what I want it to, I need to be able to get my hands on it, figure out what makes it tick, and rebuild it how I want to without having to worry about it making sense to anyone else or getting approval from how they'd do it. More than anything, it's important that we let each other have the freedom to do what we want, when we want to--if I don't see that it's going to have any real impact or I don't think it makes any practical sense, I'm not going to do it."

ISTP is an often poorly understood type that embodies a number of seemingly contradictory characteristics and seems to confuse a lot of people. It's been suggested by some that ISTPs are so different from INTPs that they should not be seen as even having the same dominant function at all--but I contend that there quite a number of similarities between the two in principle, even if the outward expressions of these principles are approached in very different ways.

Above all, dominant Ti values a highly refined sense of universal correctness, fairness, and internal consistency. While ISTPs may not make this belief as overtly obvious as their oft-argumentative INTP cousins, they ultimately believe that fair is fair and there's no getting around the idea that some things are inherently more fair, more consistent, and more reasonable than others. The thing that makes this difficult to discern is that ISTPs are, by far, the least interested in debate of all four xxTP types. They'd much rather actively demonstrate their principles and ideals through concrete action than spend time sitting around trying to convince other people that they're right. While they may easily grasp the reasoning behind various abstract representations of logical reasoning (especially when tertiary Ni is developed), they simply don't see any reason to talk about it when they could be creating, building, or participating in something that generates realistically tangible representations of the structural and symmetrical relationships that fascinate their sense of global systemic consistency.

Inconsistent reasoning and poor logic irritate the ISTP just as much as they do the INTP; the ISTP is simply much less concerned with using abstracted hypothetical explanation to demonstrate why. Actions speak louder than words. Why should he bother with empty words and arguments when he can simply show you demonstrably what it is that represents the personal sense of structural completeness around which his values are centered?

Fiercely independent, resourceful, and self-reliant, ISTPs will tend to disregard or ignore outright any rule, law, or external expectation that doesn't fit their internal set of principles regarding what's inherently fair and reasonable. They are characteristically skeptical of any external attempt to compel them to behave in any particular way, as they feel that often the people designing and imposing these rules are neither logically-minded nor genuinely experienced in the areas of life that their frivolous rules and laws will impact most. Very few things upset the ISTP's core sense of fairness more than unreasonable attempts to restrict his freedom of action or impose the will of others upon his own.

Like most Ti types, ISTPs tend to have an interest in systems and the relationships and frameworks that make them fit together the way they do. They will pour extraordinary amounts of time into the study of these systems, but rarely through book study, never without hands-on experience, and not necessarily because they accomplish any particular goals--mostly just because internalizing and possessing complete understanding of all the variables that make up a complete system is inherently satisfying on its own.

Auxiliary: Extroverted Sensation (Se)

While INTPs will tend to apply Ti's structural curiosity to more hypothetical or theoretical systems like higher mathematics, philosophy, or programming, the ISTP's dominant Ti tends to filter through Se to produce an interest in physical and mechanical systems that can be observed, experienced, and demonstrated through tangible physical processes. ISTPs are fascinated by how things work, and they want to be able to hurl themselves head-on into the full experience of how those things work by getting their hands on them and associating present-moment physical sensations (Se) with an ever-growing sense of universal truth (Ti) about how the pieces of the puzzle fit together. They tend to specialize in areas where they can use their keen present-tense awareness of sensory cues to bolster their natural ability to deconstruct and solve structural puzzles: ISTPs are often involved with auto mechanics, building/repairing electronics, designing and building architecture, performing or recording music, and so on--anything where they can get their hands on it and figure out for themselves how it works, then use that to make it do something interesting or exciting.

In addition to this, Se tends to promote an interest in the physically thrilling, high-adrenaline activities often recognized as a calling card of SP types. Indeed, ISTPs seem to take great satisfaction in understanding the variables that relate their quick sensory responsiveness to the underlying structure of the tangible, physical world around them, to the art of kinetic movement itself and how it creates reactions from the external sensory environment. It's easy to see why so many of them enjoy building and working on cars/motorcycles/aircraft, audio/visual equipment, guns/swords and other weapons, or musical instruments--these are the very objects that grant them the exciting experiences (and accompanying opportunities to practice their sensory responsiveness) that make them feel most excited and imminently alive.

But Se also serves another very important purpose: Connecting the ISTP to a real sense of what will impact other people's tastes and impressions in an immediately recognizable and universally understandable way. Well-balanced ISTPs are almost invariably "the cool guy/girl" in their social groups--they know where to be, what to wear, what to say, and how to say it; more importantly, they are confident enough in their natural talent and adaptability in these areas that, unlike ESTPs, they are keenly aware of how little they actually have to say in order to maintain that impression.

The peculiar relationship between Ti's desire for obscure systemic knowledge and Se's desire to maintain an image of smooth, nonchalant, effortless awareness of what's current and desirable leads to a rather fascinating conflict for many ISTPs: Constantly caught between "Ti nerdy" and "Se cool", they straddle the line between different worlds.

One of my favorite of examples of this phenomenon comes from the short-lived TV series Freaks and Geeks, where James Franco's ISTP character, Daniel Desario, is forced to join his high school's AV Club as punishment for pulling the fire alarm to get out of taking a test. At first he is angry about the news: AV Club? Isn't that a bunch of nerds? Oh God, nobody can possibly keep thinking I'm cool if I'm stuck doing this kind of boring crap (Se)! But later in the episode, another character sneaks into the AV room after hours and discovers Daniel in the back with the movie projector, poring over a schematic diagram of its functions and experimenting (hands-on, of course) with how it works. No matter how uncool Se may say AV Club is, he can't escape Ti's fascination with the inner workings of a complex mechanical system.

As ISTPs develop increasingly stronger Se, their awareness of others' immediate impressions of them combines with an uncanny knack for reading body language to grant them a surprisingly proficient awareness of the motivations of others. While they may not be able to expressly articulate what another person is thinking or planning, they often show exceptional talent with "gut feelings" that someone is not genuine, or is dishonest, or not confident, or hiding something. As a case in point, Doyle Brunson (that old guy with the cowboy hat you see playing poker on TV), often credited as "The Godfather of Poker", seems almost certainly ISTP in his exceptional command of body language and the astounding accuracy of his instinctive assessments of opponents at the live poker table.

On the downside, ISTPs can suffer anger issues with people they see as illogical or wrong-headed, and Se may lead them to physically demonstrate this anger through direct threats of physical violence, or worse--actually following through with them. Because the ISTP needs to be building his own understanding in order to continue with an activity, once he feels he is no longer learning or experiencing anything of value, he may become highly unreliable and abruptly drop out of projects or leave important obligations unfulfilled. The ISTP's polarized energy level may shift wildly from extreme excitement/frenzied action over a new and exciting activity to lengthy periods of non-productive burnout and back again, which can make her seem totally inert to outsiders who have not yet witnessed her in the active, excited phase.

This may lead to a tendency to drop people, things, groups, and interests as soon as they cease to generate immediate fulfillment--and as the ISTP is fundamentally introverted, he may not care at all how others perceive this inconsistent dedication/difficulty with commitment and may seem to abruptly disappear from all areas of life for weeks or months at a time, before randomly showing up and jumping back in as if nothing had ever happened. Most of my ISTP acquaintances are people that I see frequently for several months, and then not at all for several more--I've come to realize they don't mean any offense; it's just the way their cycle works.

Tertiary: Introverted iNtuition (Ni)

Interestingly, despite their (somewhat accurate) reputation as apathetic loners, ISTPs typically feel strongly enough about their principles that they may even resort to vigilante justice in order to set things right with the world when legitimate establishments of law enforcement have failed to produce a just or consistent result. They're typically quite mindful of not stepping on other people's toes, but if you step on theirs, be prepared for a swift and unexpected backlash. Somewhat like INFJs, who share all of the same function attitudes, ISTPs in the grip of Ti+Ni may fantasize about using their superior strength and physical prowess (Se) to take revenge on people who unjustly bring harm to the innocent. "Eye for an eye" is often seen as the purest and most physically real affirmation of the sense of justice that factors so heavily into the ISTP's personal value judgments.

When undeveloped, tertiary Ni most frequently manifests itself in terms of semi-paranoid distrust of "the man" or of authority figures or anyone who may have the power to force the ISTP into any situation or role from which he does not have the option to escape when he wants to. This kind of cynicism may even lead the ISTP to claim some sort of supernatural foresight; he may doggedly insist that he "just knows" something to be the case despite total inability to explain why or display any physical evidence thereof. He may come to believe anything he does is simply playing into exactly what some unseen, evil "puppet master" of sorts wants him to do, and thus may insist that the situation is hopeless because he already knows how it's all going to end up.

When applied more positively in a more developed state, tertiary Ni should bolster the ISTP's natural fluency with sensory cues by giving him deeper insight into the symbolic or suggestive meaning of the constant flow of outward sensory information he is normally attuned to. Rather than simply note what is and move on to noting something else that is, he will begin to consider the assumptions inherent in the set of rules he assumes must govern the way he interprets and evaluates that information, which, in time, will grant his "gut instincts" far more substantial meaning and accuracy by linking them to a deliberate purpose with much more far-reaching implications.

Ni should ideally help the ISTP to feel even more completely free: she will realize she can adapt not only her present actions, but also her entire outlook and attitude according to whatever the immediacy of the moment demands: nothing can ever shake her composure because she can simply change her mindset to fit her surroundings.

When Se is poorly developed, and a TiNi loop results, the ISTP loses all desire to connect or exchange information with others in any meaningful way. He becomes extraordinarily self-centered, acting out his subconscious Se desires through increasingly impulsive (and possibly even dangerous or violent) outbursts, insistent that he is the only one who "has it all figured out", that all the small-minded fools surrounding him are running a pointless rat race for nothing, and that he does not and should not ever have anything to prove to such unworthy and insignificant creatures.

He may delve further and further into bitter cynicism and conspiracy theorist behavior, as Ni develops increasingly outlandish interpretations to justify Ti's all-important desire to view the self as the only remaining bastion of consistency and truth in a purposeless world that cares for neither. Nihilism and hopelessness invariably result.

Ultimately, tertiary Ni should grant the well-balanced ISTP a unique sense of worldly wisdom. Already generally subdued by nature, his calm, quiet confidence will be nearly unshakable, as he will find the ability to separate himself from the tribulations of everyday problems enough to realize that eventually, everything is going to be fine, so we may as well just focus on dealing with what's in front of us and trust that everything else will fall into place the way it's supposed to. Development of tertiary Ni often coincides with a time in the ISTP's life where he retreats into a period of serious self-reflection and emerges with a far more calm and stable sense of philosophic purpose and global awareness.

Inferior: Extroverted Feeling (Fe)

As the weakest point in the ISTP's cognitive hierarchy, Fe presents some substantial issues for the "lone wolf" persona in which he so often finds himself entrenched. Insistent on figuring things out for himself and living life his own way, the ISTP is prone to ignoring his emotional and interpersonal needs as long as he can possibly get away with it. He may become increasingly stressed as he realizes that without some form of permanency or obligation, some sense of connectedness to a group or purpose larger than himself and his own personal needs and desires, he feels as though his life is simply running in circles and never reaching any meaningful conclusions.

Inferior F types (IxTP, ExTJ) tend, by nature, to be utterly clueless when it comes to expressing or even acknowledging their emotional needs, and may resort to displaying them through rather bizarre, confusing, and even childish behaviors. For most ISTPs, Se serves as the only comfortable link to the external world, the only way they understand how to bridge the gap between their internal ideals and the expectations and aesthetics of others. Thus, Fe is often expressed in a way that becomes slanted by Se's tendencies: ISTPs care a lot more about their family and friends than they are able to express verbally, and since actions speak louder than words, in moments of extreme stress, they may demonstrate their cultural and familial obligations by physically attacking or forcibly removing anyone or anything which threatens the sense of moral fiber upon which their families, social groups, or communities are founded.

This may simultaneously impress and disturb others, as friends and family are often surprised to see that the ISTP even cares enough about them to do anything protective in the first place, but also upset that s/he chooses such directly physical means of expressing the importance of his relationship to them. The ISTP may often be seen as emotionally unaware and even incapable of emotional expression, and while she may resent this suggestion, she often does little to nothing to counteract it until the perfect moment arises: in a flash of daring bravado, a selfless act of unexpected chivalry or intense but unexpressed loyalty will allow the ISTP to release the build-up of subconscious tension and guilt over her insensitivity to the collective needs and ethical expectations of her loved ones. She simply cannot respond to or consciously acknowledge these needs easily because they seem to threaten the sense of individually-defined identity and freedom that she holds so dearly.

The central conflict for ISTPs struggling with inferior Fe is their insistence upon absolute personal freedom at all times, and the seemingly disingenuous nature of participating in familial or cultural ritual when dominant Ti can't see any reasonable or logical purpose for it. Accepting objectively derived concepts of morality or interpersonal obligation threatens the prized ability to change or escape any undesired situation on a moment's notice. The ISTP feels threatened by expectations of others upon him to behave in ways he does not find reasonable; however, he must confront the fact that he does require some relationships with others to feel completely fulfilled, and that eventually most people will tire of his simultaneous expectations to be accommodated and refusal to accommodate their needs in return.

Again, it's most important to remember that for ISTPs, actions speak louder than words. Expecting them to verbalize their feelings on a regular basis is likely to meet with condescension and resentment; however, leaving them enough space to do as they please will almost invariably produce enough appreciation that, in time, their occasional shows of good faith will develop into a more complete awareness of the needs of their friends and family, as they will gradually realize that coming to collective moral agreements for defining and strengthening interpersonal bonds actually makes all the logical sense in the world.

Learning to accept and embrace some degree of cultural expectations will grant the ISTP both a more objective method of self-evaluation, and a much more balanced sense of consistent reliability. (It also doesn't hurt that, as they get older and social expectations change, accepting more responsibility actually makes them look a lot cooler and more aware of external reality!)