Fi is defensive about deeply held beliefs and the validity of its perspective. It tends to lash out when central beliefs are dismissed or attacked. Fi also tends to get upset when the personhood of others isn't respected.
If Ti hates idiots, Fi hates cruel and/or dismissive assholes.
When attacked, Fi usually retreats or questions the emotional/moral integrity of the attacker. If pushed, may use Fi-based insights to go for the soft emotional underbelly of the attacker.
This is a good summary of the Fi side.
I see your point. They're both defensive on their own subjective subjects.
Yes.
I think of like a wild animal cornered or something. It feels under attack. Vulnerable.
There is something to this. I think it hinges on functional attitude.
The four introverted functions are Fi, Ti, Ni and Si. Each and every one of them is defensive. Trying to convince someone relying on these functions of anything is often futile: the best tactic is to appeal to a complementary extroverted function.
Fi gets defensive about feelings. It is nigh-impossible to verbalize feelings for this function, and one often has to rely on Te or Ne or Se to do so. Each venue slightly garbles what Fi really is saying.
Ti is very similar. It will absorb knowledge and information like crazy, but any attempt to
force such absorption is resisted. When asked what they think, it can come out negatively through Fe (opinionated), Ne (scatterbrained), or Se (not sure of a good example for this one, perhaps some degree of violence). The main thing is that like Fi, it is not easily verbalized.
Ni, even though a perceiving, rather than a judging function, has elements of the above, too. It is perhaps not as obvious an example, because Ni-doms are rare, and since it's a perceiving function, one finds oneself interacting with Te or Fe, not Ni. However, when an extroverted function is unavailable to translate Ni, Ni becomes very confused, almost cassandraesque, able to foretell doom, but no one will believe because there is no demonstrable
reason to believe. It is very difficult to disabuse an Ni-dom of his or her intuitive perceptions. There is this internalized feeling that whatever it is they intuit, it
has to be true, they just aren't sure "how" it is true, yet.
Similarly, Si hides behind Te and Fe, but is a bit more prominent as Keirsey noticed for temperaments: SJs are all Si dom/aux. What I've noticed here is that aside from the typical Keirsey SJ observations, Si is
very sure of one's own internal representation of reality. For lack of a better word, call this "memory." There is a tendency of Si to "remember" things that aren't actually true, and it can be difficult without hard evidence to convince them otherwise. This is not to imply that dom/aux Si users don't admit to not remembering certain things, but rather, they are very very sure of what they do believe they remember. I was married to an ESFJ for several years. Her memory was astounding and I often relied on it. When she started using it as a weapon against me, I noticed its flaws: mainly that she seemed to have an internal construct with its own rules, and some of those rules didn't jibe with reality, so she wouldn't remember things that didn't concur with the rules. (This is, of course, a somewhat biased point on my part ... I'm not trying to make Si appear bad, but rather to express "how it fails when it fails".)
In all of these cases, the essence of the disagreement, the reason of the defensiveness, is all introverted, internalized and virtually excluded from verbalization. The other functions try to verbalize it, but fail, leaving out key elements of the introverted understanding. The result is "defensiveness."