As an INTP, I can describe what my own personal Ti looks like. Being that it's my primary function, it's what I want to do most. It's my "default" mode. When I've been around people all day and interacting, I want to go home and tune everything out and think deeply on a subject, to analyze some sort of system (social, corporate, amongst many). I am able to devour huge amounts of information in short periods of time. For instance, if something happens in my life that is painful or that sets me back (or if I'm just deeply curious), I can go to the bookstore, purchase a good book or two on the subject and just inhale the information until I feel that I have it almost completely understood. The older I get, the more I realize that life has to be lived rather than just "read about in a book". I'm only stating this to describe what the Ti looks like (at least in my world). It's like a deep desire or instinctual need to learn just about everything that I can about something (and often as quickly as humanly possible) until I'm comfortable with my own understanding/competency of that subject. Then I feel like I've got my head around that, I understand it quite well, and I'm on to another venture. It's quite fun...at least for an INTP. But, people who I interact with know me for my Ne. That's what they see. For the most part, they have no clue of what goes on in the Ti realm.
But, I have no clue what Fi (as a PRIMARY function) really looks like or how it operates. ISFP's/INFP's care to comment or offer some insight? Is it similar to what I described but more focused on reflecting on/running things through your own personal value system? What about in relationships? Are you spending your time (when in Fi mode) "feeling" if it's right to be with this person or "feeling" if they really love you? Just curious as to how that inner world works.
A curios INTP