People mistype themselves all the time. INTJs sometimes think they’re INTPs, and vice versa. Want to know how you tell the difference? Ask a question that can be solved with provable facts and propose a crazy possibility. The INTJ will quickly work through the problem internally and conclude that theory as illogical; the INTP will discuss it out loud, coming up with arguments for and against it almost at the same time. They will state a theory that supports it, then turn around and say, “On the other hand, you could argue this to support the opposing side…” They will pick holes not only in other people’s logic, but their own, sometimes undermining their previous argument in the second half of the sentence. INTJs don’t have to deal with that — they just rationalize it out in their heads and reach a conclusion.
The INTP is reluctant to reach a “final” conclusion, because their Ti rationalizes that there must be information out there they haven’t considered yet, and goes searching for it using Ne. The function of extroverted intuition (or Ne) is like handing someone a sheet of paper with the barest outline of a tulip on it. The Ne user takes that tulip and fills in all the colors, then adds more, until it’s an endless garden not just of tulips but sunflowers, roses, and stinkweed. They tape the image to the wall and keep drawing — it becomes a field with a castle on one end and a dragon on the other. Ne is an outgoing force – it continues getting bigger and bigger, evolving into new ideas and building outward into new possibilities. It sees no end to the information, therefore it can reach no “definite” conclusion except through social rules (Si) and faith-or-socially-based morality (Fe). Ne won’t ever stop until it either knows everything there is to know about a subject, or is reigned in through force of will.
If you sat an INTP down and asked them a bunch of various zany questions, they’d have a lot of “maybes” and very few “certainties.” Do ghosts exist? No? Maybe? Do aliens exist? Maybe. God? Possibly? Or Not? INTPs may or may not believe in something, but they won’t tell you that their decision is final, because how can it be final, when there is an endless stream of possibilities, information, arguments, counter-arguments, etc., to filter through before reaching any firm conclusion? Their Ti-Ne requires constant re-evaluation of previous information when and if new possibilities are introduced.
An INTJ, on the other hand, will state with certainty what they do and don’t believe in, and give logical reasons why based in real-world evidence. INTJ’s Ni works differently from an INTP’s Ne — it sees the tulip, constructs the field, flowers, dragon, and castle, and then uses their Te-logic to discard the outrageous things and find the ruby hidden underneath the petals of the original tulip. That’s a focus point, a conclusion, the logic kicking in and filtering through the ideas to concentrate on the most useful part of the field. Ni has just as much imagination, but it all takes place inside their own mind (as opposed to the INTP arguing it out externally), so all you see is the conclusion. The INTJ thinks about it (silently) and gives an answer. The INTP argues it aloud (or in writing) in real-time, as it happens. Ne needs to go outward; Ni focuses inward.
The easiest way to understand how Ti and Te impact the process is to compare it to the conflict in the Harry Potter books between Hermione (an ISTJ, Te-user) and Luna (an INTP, Ti-user). Hermione finds her logic in the external world – in the books she reads, in the information she has learned, in things you can actually prove, which is why she scoffs so much at Divination. She is very much a doer who launches into an action — even though she isn’t an INTJ, INTJs are also “doers.” It’s not enough to think about it, one should act on it! INTJs process the information, then act on it — write that book, go on that trip, whatever. Te is all about action in the real world, about doing things or implementing change (like Hermione seeing a need among the house elves, and starting up a group to help them).
Hermione’s realm of logic is completely different from Luna’s, who finds her logic internally and doesn’t like relying on external facts to form her conclusions; even if you can prove something is wrong to Luna, she’ll still believe it’s possible. She believes Harry is telling the truth not because he can prove it, but because it’s within the realm of possibility and it seems logical to her. Luna isn’t all that “active,” because she’s less of a “doer.” She’s more interested in internalizing information than doing anything with it — she spends time getting to know the Hogwarts ghosts not for any real reason, just because she’s interested. Ti is more about learning than doing; it’s not as interested in implementing change or taking action. It will take action, but only if it must or it is excited through Ne (like writing a book — it’s an adventure, possibilities waiting to unfold).
Both of them are incredibly smart, but Hermione’s conclusions are final and she gets things done – Luna’s are more dreamy and she’s less driven.
P’s have a reputation for being indecisive because of their endless stream of possibilities. They spend hours researching both sides of an argument, only to reach the end and realize they haven’t made up our mind, because both sides make good points. That means they’re back where we started! True, now and again they do decide something and stand on that with firm conviction, but only if they’ve exhausted every possible alternate theory and concluded that there is no additional evidence to add to the information stack… like, say, conclusions based on historical events or individuals.