Since I’ve already done endless posts on Ne and Si, I’m going to leave that portion of this essay out. By now, you should know what Ne and Si do together (create ideas and bounce them off former knowledge).

It can be hard to tell these two types apart on the surface. Both of them tend to be quiet and withdrawn, but highly imaginative and creative. The biggest difference is that the INFP completely knows how it feels about any situation and makes emotionally based decisions, while the INTP doesn’t know how it feels about most situations (and can’t understand or explain their emotions if they need to) and makes logical decisions.

For example, Cosplaying: the INFP will get an idea and go do it. Their introverted feeling (Fi) knows what it wants (to be this character) and their extroverted thinking (Te) enables them to act on that desire to make it a reality. There is no real emotional angst, no real indecision, just action. The INTP will get the same idea and then talk themselves out of it. Where would they wear it? Is it worth the cost? Would they enjoy owning it? Is it practical? Do they even truly want it? Is that costume practical for the event they would wear it to? Is it too hot for summer? Too cold for Halloween? Their introverted thinking (Ti) goes to battle against their extroverted feeling (Fe), which throws more complications into the mix – what will others think of them for cosplaying or spending money on cosplaying? Fe asks them to get external input — is this a good costume idea? Would it suit me? Do you think I should buy it? In the end, the INFP has their costume and loves it. The INTP either doesn’t have a costume and half-wishes they had one, but every time that desire kicks in, their logic shows up to point out all the reasons they don’t need it, or they bought the costume and are a little ashamed of owning it, because it was an “impractical emotional” decision.

INFPs are very driven to be “true” to themselves. It’s highly important to them that their uniqueness and individuality be recognized and respected. They resist anyone trying to conform them to the external world, because they have total confidence in their ability to make their own decisions, based on what is right in the situation for them and how they feel about something. They are so private with their emotions that they don’t often come out until conflict arises – then, outside forces are shocked at how forceful and “stubborn” the INFP can be in sticking to their guns. INFPs seek to give everything in their life meaning, and to identify with others who share their desire to be true to oneself. They often adopt a particular cause that has great personal meaning to them and champion it throughout their life.

Fi influences everything they keep and do – they know exactly how they feel about an object or past experience (Si) and can tell you why they’ve kept it. Their Te is visible in their ability to easily articulate their feelings (when they choose to share them) and pursue what they want. Like most Te users, they place their trust in provable realities and external sources of logic, but can be limited by the facts of the situation. Because their Te is an inferior function, the INFP is insecure about it and not always confident in using their rationality to make decisions. 

INTPs have a similar drive toward independence, but it’s based on their trust in their logical approach to life rather than any desire to be “true to themselves.” They care more about being recognized for their intelligence and ideas. Their natural default is to be detached and objective in any situation, and they can bypass their feelings when their desire to learn and understand overcomes their disgust or sadness. They have no respect for emotional decisions – even though they may come to see the reasons behind it, they can’t wrap their mind around making a decision purely based on emotion. Their Ti won’t let them! “You bought that expensive costume, and you can only wear it once a year, out in the hot sun at the Renaissance Festival? Why? You can’t wear the costume at Halloween, because it’s too skimpy! You’ll get cold!”

Logic is important to them, so they stubbornly stick to it – at times, to the point of obnoxiousness, in part because their Fe wants other people to validate and agree with them. Fe makes them accommodating on everything they don’t have an opinion on or care about. Because it is such a weak function, the INTP is self-conscious about it and only partially uses it – they want to keep the peace to avoid emotional fall-outs they don’t feel qualified to handle, and it eats them up with guilt if they make a selfish decision that hurts others. Their own emotions frustrate them, because they can’t rationalize, understand, articulate, or at times control them — the latter is particularly frightening, since it means a total departure from what they know best: logic. Fe gives them general concern for humanity (as opposed to the personalized concern of the INFP) and an impartial ability to argue both perspectives, but most of the time the INTP doesn’t know HOW it feels, how it SHOULD feel, or even how TO feel. Their occasional emotional outbursts shock others, because the normally pliable, laid-back INTP can have a sudden temper tantrum or fit of irrational stubbornness. 

They also keep things out of sentimentality, but have no idea as to the reason. It’s an emotion and they can’t explain it or understand it. Often, INTPs appear warm on the outside (their expressive Fe makes them animated, helps them pick up on conventions and accommodate others) but consider themselves to be cold on the inside, because they can’t muster up that incredible emotion other people seem to experience naturally. They are aware of their utter incompetence when it comes to “genuinely caring” about people. Where the INFP truly attaches to others and only expresses affection when they mean it, the INTP feels an emotional distance from others and mimics emotions to fit in.

Both, in a way, feel isolated from others – the INFP in their emotions and the INTP in their lack of emotion. The easiest way to tell them apart is their assertiveness and convictions; the INFP will stand firm and defend itself if necessary; the INTP will accommodate and even at times deflect self-defense… because it’s not sure how it feels, or how to express those feelings. The INFP will dream freely but sometimes make foolish or selfish decisions based on emotions. The INTP will crush their dreams with logic but sometimes neglect their own needs in their desire to keep others happy.

"Bad" behavior from an INFP includes selfishness and whining about their situation; "bad" behavior from an INTP includes passive-aggressive behavior and resentment toward others for not providing for their needs. The INFP assumes everyone else is as passionate as they are, and the INTP doesn’t understand why everyone isn’t as logical as they are. Both have wonderful strengths (a vivid imagination and excitement for ideas), but the INFP does best in an environment catered to its ability to feel and dream, and the INTP is happiest when allowed to be playfully logical.