There are philosophers who maintain that morality is a function of, or reducible to, rationality.
It is impossible that they could be right. Natural selection would eliminate individuals who were “rational” to the extent that being “rational” differed from believing and doing whatever would tend to maximize their genetic legacy, provided that they could believe or act “irrationally.” Since believing or acting “irrationally” amounts to being able to do wrong, and since even “rationalists” know that people can do wrong, the condition is fulfilled. Therefore, it is impossible for the “rationalists” to be right.
I suspect that the impossibility of their project explains why, amidst their pirouettes of pointless cleverness, it is frequently possible to find elementary fallacies such as equivocation.
I suppose that I really ought to put scare quotes about the word “rationalists” in my title.