How and why we reason from is to ought

Originally identified by Hume, the validity of is–ought inference is much debated in the meta-ethics literature. Our work shows that inference from is to ought typically proceeds from contextualised, value-laden causal utility conditional, bridging into a deontic conclusion. Such conditional statements tell us what actions are needed to achieve or avoid consequences that are good or bad. Psychological research has established that people generally reason fluently and easily with utility conditionals. Our own research also has shown that people’s reasoning from is to ought is pragmatically sensitive and adapted to achieving the individual’s goals. But how do we acquire the necessary deontic rules? In this paper, we provide a rationale for this facility linked to Evans’s framework of dual mind rationality. People have an old mind which derives its rationality by repeating what has worked in the past, mostly by experiential learning. New mind rationality, in contrast, is evolutionarily recent, uniquely developed in humans, and draws on our ability to mentally simulate hypothetical events removed in time and place. We contend that the new mind achieves its goals by inducing and applying deontic rules and that a mechanism of deontic introduction evolved for this purpose.