The bolded is the Achilles' heel of Ti. Is one's subjective impression of what is true based on personal preference or a desire for truth in spite of not liking truth? Subjectively, it's actually difficult to differentiate. Einstein really didn't like quantum mechanics in any of its formulations, in spite of his contributions to the field, because it didn't fit in with everything else he knew to be true. There is always, in the end, a subjective certainty that certain things are true, and in a rigorous Ti framework, this necessarily precludes other truths that simply don't fit in the framework. 99% of the time, it doesn't matter. The remaining 1% of the time, there is something that needs unlearning.Both can be somewhat subjective, I think (depends on how you spin logic?), but I think that Ti seems more concerned that it is not just "right" for them, but that it is right from a measurable and provable standpoint. (Perhaps their measure or their starting points can be a little subjective though?) Accuracy of thought is very important to Ti users, which is why they often tend to care more about whether what the other person is saying is true over whether or not they like that truth. I also think it is why Ti users tend to use a lot of qualifiers in the the statements they make. My Ti perceptions may be different though than a dom or aux Ti user's.
Note PB's precision in discussing Fi matters. She doesn't give up on trying to demonstrate truths, but it's difficult for those who aren't already on the same page to catch up and understand what she's getting at. And while these truths might be described as "values" and "subjective," they can be every bit as true as a Ti-style logical framework. At the highest level, it isn't about feelings or idealistic principles, but wisdom, the truths that don't reduce to logical syllogisms.On the other hand, I think Fi would also argue that the conclusions they come to are also The Truth. However I think they would be more okay with knowing that truth, without worrying about how to prove it to someone else in a quantifiable manner. It is about finding truth in matters of feeling and that is difficult to conclusively prove to anybody but oneself, even if the Fi user feels that the principle is more universal than that. I have a harder time talking about it from this perspective, simply because I'm not in their heads, so if I'm grossly mistaken, my apologies.
(Fe has a similar access to the same Fi truths, but apply them in a more hands-on practical way.)