As far as INFPs go, Fi works with Ne as a sort of checks and balance. Ne is sort of the tunnel to the external wold where we perceive info, and Fi assigns value. In this manner, we don't arbitrarily form values, but it's after much consideration of the patterns we've found and the connections we've made. So Fi has a framework built over time, just as Ti does. Our framework is our set of ideals, and this again comes via Ne, which can see what is not there, but what could be, based on what is there.
So Fi is a rational thought process, but because it is values based and considers emotion, it is subjective. Ti is objective in the sense that it's separated from emotion, but it's still an individual's subjective view.
Because Fi is subjective, it means we look beyond the black and white logic, and include ethics and emotion as variables (this is why we have strong compassion). If something brings about the desired outcome, it may be logical, but if it violates a value, then it is unethical, and therefore it is wrong, even if it makes "sense". Emotions can guide the judgments Fi makes also, such as determining something is bad for yourself because it makes you feel bad, and that's also why we accept individual values (what's good for one person is not good for everyone).
However, Fi can operate separately from emotion because our ideals can clash with our emotions. Since Fi is rational, we can recognize when our emotion is not and then go against our emotion.
Other times, our ethics and/or emotions can distort our perspective and that's when Fi seems illogical, because it's gotten lost in its subjectivity. We may have assigned so much value to something that we lose sight of what is actually more important in reality.
We're selfless in the sense that we seek internal harmony, but not just in ourselves. The goal is internal harmony for everyone. Fe can seem "surface" to us because it's so group-oriented, whereas we are individual-oriented. Fi will tolerate getting messy externally if it means cleaning things up internally. That's the way we heal, and also why we don't value social etiquette as much as personal authenticity. Strangely enough, this makes us more universal in our feeling, because there are no groups, but rather just individuals.