From: Philip Johnson-Laird BA PhD Psychology (UCL), Stuart Professor of Psychology Emeritus at Princeton. (Author isn't a logician.) How We Reason (1st edn 2008). p. 275.
Do Kpelle rice farmers in Liberia reason in the same way as villagers in remote Uzbekistan? Do Asians reason in the same way as Europeans? Do we reason in the same way as our ancestors? Social scientists argue about these questions. Some say that human beings reason much the same the world over and always have done so; their opponents say that human beings differ in how they reason depending on their culture. Some argue that the norms of rationality should be the same for all cultures; their opponents—so-called “relativists”—argue that norms should differ and depend on the local culture. Hence, what is rational for the Azande of Central Africa is irrational for Evans-Pritchard, the English anthropologist who studied them, and vice versa. It is hard to resolve controversies about norms, but the educability of peoples from all over the world suggests that no profound differences in reasoning exist from one culture to another. The debate has often been couched in terms of formal rules of inference. No formal rules are universal, relativists say. But, their argument presupposes that we use formal rules. From the standpoint of the model theory, the question to pose is: do all cultures realize the force of a counterexample? Show me a culture that does not, and I will concede that relativism is right for some cultures, though not for mine.
Relativism has an aura of self-refutation (like my last remark), because relativists must allow that there is a subculture of rationalists, who believe that the principles of reasoning are universal. Suppose relativists claim that rationalism is right in its subculture. It follows that the principles of reasoning are universal, because that’s what rationalists believe. And so relativism is wrong. Suppose rationalism is not right in its subculture. It follows that there’s a culture whose principles of reasoning are not right, and so relativism is wrong. Either way, it follows that relativism is wrong.
Why's the emboldened sentence true?
I understand that relativism is self-refuting, but for this different, more straightforward reason:
- All truth is relative.
- If all truth is relative, then the statement "All truth is relative" would be absolutely true. If it is absolutely true, then not all things are relative and the statement that "All truth is relative" is false.