There's actually a point when rational thought becomes irrational, when the person moves so far away from facts and experiences into rationalization that they're entirely detached from anything that's real.
In fact, a person can rationalize themselves into poor mental and emotional health, by declaring that varying things like fun, relationships, and creativity aren't "rational."
It's why I hate nihilism. It seems like going full-circle from being rational to complete emotional retardation. There's a point where rationality becomes stupid.
That's why we have both a left and right brain.
Still, my argument in this thread has been that what is "rational" includes ethical value systems and human structures of society that serve rational purposes, though they may not fit a detached, Ti system of what the Ti user personally finds rational.
I think that's why it's crucial to realize that Fi and Fe are rational functions, though they are not logical. All judgement functions are rational.
Furthermore, Ti isn't a fully objective function. It's SUBJECTIVE objectivity, so it contains an unfortunate element of applying rationality to a very personal, individualized subcontext. This is very useful in picking things apart into very small pieces and deconstructing very particular ideas...but if the basic foundation for a Ti rationality is wrong, even if the logic is perfect, the entire house of card FAILS. Te is objective objectivity so it is more objectively logical than Ti, but lacks Ti's depth and precision.