INTJ and INFJ are perceiving and observing things first-and-foremost, which leads them to grasp information in their surroundings. The INTP and INFP are looking inward towards their own understanding of the world and the principles correlated towards them.

INFJ’s overlay their imagination onto reality like a set of architectural plans and tracing paper; tracing over whatever their deductions and bleed onto and organize.

This is exactly why Ni-dominants make the most natural inspectors or detectives, counselors and such. It’s precisely the fact that they work with context first-and-foremost, by a case-to-case basis that they can creatively come up with effective ideas and solutions tailored to the problems at hand, right to specific details.

In this sense — every situation is subject to nuance and by no means are any two situations exactly identical.

For the Ne-auxiliaries (INTP and INFP), unlike the previous two INs, rather than deconstructing the very subject they perceive — they look inward, towards their own understanding, rather than a specific subject or idea they’re inspecting. The most concise way to dig through their knowledge without over-complicating or tripping over precise technicalities is by reducing their entire database to a set of principles.

And an INTP or INFP will look deep into their imagination and look inward. In a sense — if an INJ were to overlay their imagination onto reality, an INP does the reverse; overlaying reality onto their imagination/understanding instead.

So they tend to do a lot of cross-referencing.

Think of them as scholars and librarians.

Naturally, if a person retreats into their heads and looks for answers internally, they redirect their focus and shut out sensory imagery in order to flip to mental imagery, which essentially explains why they’re often giving off blank stares — mainly because they’re not really using their eyes.

This is why INPs make excellent theorists and visionaries.

So the stark difference between INJ and INP imagination would result in how their eyes sharpen/dull.

Because an INJ when using Ni uses their eyes to observe — essentially dialing up the use and focus of the subject they examine. This is why they give off very piercing stares.

Whereas an INP dulls their eyes because they’re looking towards their internal library — by redirecting their focus inwards, they block out a lot of sensory information in order to concentrate.

—Timothy Emmanuel Lim

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A comment to the post:

As others have said, the blank expression works for me as an INFP, in fact, I do a lot of my “thinking” at night while “asleep” and my eyes closed. Moreover, I don’t just turn off my eyes, I turn off my ears. If you talk to me while I am thinking (like writing here on Quora or programming), you will find that I don’t usually respond right away, and when I do it is with annoyance that I had to break flow and “turn my ears back on” and then ask you what you said, because I only caught a couple of words of it toward the end and I haven’t pieced them into a coherent sentence—and I literally want you to repeat what you said word-for-word, because parts of it are still stuck in the buffer in my brain where I store sentences I’ve heard but haven’t comprehended yet.