A critique of a priori moral knowledge

Abstract

Many ethicists believe that if it is possible to know a true moral proposition, it is always possible to ascertain a priori the normative content of that proposition. I argue that this is wrong; the only way to ascertain the normative content of some moral propositions requires the use of a posteriori information. I examine what I call determinate core moral propositions. I assume that some of these propositions are true and that actual agents are able to know them. Ethicists whom I call coreapriorists believe that it is always possible to ascertain a priori the normative content of such propositions. Core-aposteriorists believe that this is false, and that sometimes a posteriori information must be used to ascertain that normative content. I develop what I call the a posteriori strategy to show that core-apriorists are likely to be wrong, and so core-aposteriorists are correct. The strategy examines the details of particular core-apriorist theories and then shows that the theories have one of two problems: either some of the knowable determinate core moral propositions in the theories are not knowable a priori, or some of the propositions are not determinate, so they cannot perform the epistemological work required of them. Therefore, some knowable determinate core moral propositions are only knowable with the aid of a posteriori information. I apply the strategy to four different core-apriorist theories. The first is Henry Sidgwick’s theory of self-evident moral axioms, as recently developed by Katarzyna de Lazari-Radek and Peter Singer. The second is Matthew Kramer’s moral realism. I then examine Michael Smith’s moral realism, and Frank Jackson and Philip Pettit’s moral functionalism. The a posteriori strategy shows that there are serious difficulties with all four theories. I conclude that it provides good evidence that the core-apriorist is mistaken, and that the core-aposteriorist is right