I use Introverted Thinking when I am reading. I automatically diagram sentences. Several operations are going on. First, each word is weighed for its full semantic range (my INFP husband weighs them for linguistic origin, slightly different), then I decide on the point within those ranges which is being targeted based on the other words that are being used and the way they lean on one another (indicating which direction the sentence as a whole is leaning.) (Here intuition comes in as I leap to probabilities, shortening the process.) Then syntax and structure are analyzed, impossible meanings are instantaneously ruled out, and the precise intention of the sentence is arrived at. If the composer of the sentence has not been precise, I use Si to recall the phrases and verbal shortcuts such people use and insert their memorized meanings. Am I autistic? Are all autistics INTP's? I do not know. However, there are texts which no one I have ever met has understood properly and it's painful to hear them nattering on about "interpretation." It's impossible for me to communicate to such people because whatever proofs I may offer will not be accepted as such by them, simply because they would have to take a course in logic to understand those proofs. And yet I am certain and they are merely opinionated. I know the difference.
I use the passive to describe this process because it feels almost passive; this is something I do because I must, whenever I glimpse a sentence. At the same time I feel that I own the process - it makes me certain of what, precisely, a sentence has been able to express and what it has not. People often think their sentences have said far more than they actually have. However, these people are considered to have said what they intended to say so long as their audience "got the gist." But of course, gist-ing can be a relief even for me when verbalization has worn me down. Ultimately, I have thoughts and translate them into words, rather than thinking in words. Language is suitable to clothe thoughts because for me, the structure of thought is analogous to the structure of known languages and I think it is the same for most people. However, my thoughts exist prior to the words and do not need them and I think that may be the difference. The thoughts form an invisible structure, a permanent one, in which the whole nature of reality is represented in my mind as I constantly sort everything that enters for its position in a hierarchy of causation and relation. However, if you want to visualize what this is like for me, the structure has floating, not very particular components which can be adjusted or exchanged at any time. It's like a 4-dimmensional flow chart of reality in my brain that somehow ends up looking like a castle from a distance; the goal is to multiply components, make them smaller, insert more connections between them, and detect similarity in patterns between completely different types of systems. Sometimes I wish I were a physicist but unfortunately, I do not enjoy math as it's the wrong type of labeling for my kind of logic. I have theories about all subjects of study but am especially interested in the range between electronics and neuro-psychology.
What is Ti? Where Fi sees invisible essence and qualitas, Ti sees invisibly necessity and consequence. Fi sees beauty; Ti sees truth. (Not fact, truth.) However, truth is a kind of radiance so it is really very easy to appreciate the INFP's viewpoint as well. I believe that none of us experiences our functions singularly, but always as a union. So an INFP would experience Fi differently from an ENFP, and so with an INTP and ENTP (or ISTP, even more so.) It's difficult to distinguish what nature refuses to separate, but of course trying is the fun of it.
When I was 16 I was reading the dictionary and I happened upon the word 'syllogism' and read the definition. The example given I have never forgot: First premise: Socrates is a man. Second premise: All men are mortal. Conclusion: Socrates is mortal. In that moment the entire structure of formal logic appeared instantly in my mind. I was surprised, later, to find that instructors wished to assure me that, had the second premise read "Some men are mortal" the conclusion would not have been logically valid, however true. That should have been obvious - it was consequent upon the whole structure, it was implicated in the very existence of - EVERYTHING! All logic can be reduced to the simple statement "IS" and thus reality proves itself and faith is not the major leap everyone thinks it is. Faith does not tell you that something "IS" - that's logic. Faith, then, is confidence in the goodness of existence as opposed to the idea that "IS" = evil.
But I wonder if other INTP's will agree with me that what we call reality or existence does not really need to take into account each everyday event, which are somehow lacking in IS-ness since they pass away so quickly. Does rigidity, to use the negative word, or permanence, to use the positive one, go along with Ti more than other functions? I suspect so.
So I think that Introverted Thinking is the process of or is related to formal logic. And what, in the subject of logic, other people must learn with labor, we primary Ti users prefer by nature. But an ISTP uses formal logic on a different kind of premise than an INTP does. Our premises, those toward which Ne inclines us, are basically philosophical. Theirs may be religious but require far more concreteness and seem to prefer physical facts to work with, such as the dimensions of an interior space or the sequence followed by an internal combustion engine (or verses of scripture as opposed to theological concepts. Are all fundamentalisms invented by ISTP's? I would not be surprised.)
Ti is different from the kind of logic which knows how people perform practical operations and which tells you how to get them done. I have been weeping over my housekeeping, and my husband, an INFP, tapped into his inferior Te to help me set up some household systems - placement of trash bins, extra laundry hampers, things of that sort - which instantly solved many problems I was having. Despite my high scores in certain tests regarding verbal intelligence, I was incapable of seeing THAT sort of consequence - the kind that relates to organizational processes which people perform.
I also think it's worth saying that Ti, like Fi, is constantly analyzing. (My husband and I love one another as much for the amazing analytical conversation we have as for everything else combined, children excluded!) Isn't that different from Te and Fe somehow? My brother in law is an ENTJ and I would say that when he goes on about something, he is describing something which exists fully formed in his mind. When I go on about something I am putting words to what I believe to be a part or a member or a contributing factor to something which I see as real and not fully possessed by myself internally.
However, I do not expect most people to understand a word I say anymore. Sigh. That is, I can say things they understand but such things are repetitive and boring. How many connections must I indicate, how many components of the internal flow-chart must I name, before they begin to see something of the picture I see? I haven't found out yet.