#titag

This is a really good question, because it is probably the main reason why so many NTPs mistype as NTJ and vice versa.

The most important difference between the two functions is that Ni is a perceiving function, and that Ti is a judging function. Ni is interested in the true essence of what something is. It doesn’t bother with rights and wrongs or if it make sense, just what it is or what it is or supposed to be. The NJ will then proceed to judge the idea/concept whenever it is good or bad on if it has real life applications(Te), or it benefits the common good(Fe). Ti is interested in the inner workings of things - or the logic behind it. For Ti there is one truth, and everything has to fit in with its logical structure. This is really important because TPs use this as their basis for making decisions, and will therefore be very hesitant to voice their opinion on something (or make a decision) if they have not fully figured out the logical structure behind it, or how it works.

ENTPs with anunderdeveloped Ti can show off in two possible ways: they either rely too much ontheir Ne or too much on their underdeveloped Ti.

An ENTP who rely way toomuch on their Ne appears as very scatterbrained and irresponsible. These kindof ENTPs are drawn toward what interests them in the moment and when it startsto bore them they immediately jump to the next thing that interest them. Theyare very stereotypically ENTP and might mistype as ENFP becauseit´s hard for them to analyze if they use Ti or Fi as an auxiliary function.These ENTPs will often use Ti to justify their actions - no matter howirresponsible they might be. 

An ENTP who rely too muchon their (underdeveloped) auxiliary Ti is obsessed with finding logicalexplanations to everything, which might be used on the wrong premises. TheseENTPs don´t recognize the logical fallacies in their own analyses. Forexample: humans can´t fly. What else can´t fly? Stones. Therefore; all humansare stones. This is a very exaggerated example, but it´s a trap many ENTPs withunderdeveloped Ti falls into and therefore mistype themselves as INTPs. Iam a prime example of this. I used to be obsessed with finding logicalexplanations to everything when I was kid. When I couldn’t find a naturalexplanation to what I tried to solve, I often jumped to a supernaturalexplanation because I couldn´t handle the answer “I don´t know”. There wasalways an explanation to everything - no matter how silly it was.