  A naturalist might accept that the open question argument works in relation to moral concepts, but deny that we can make any inferences  about the way the world is from the fact that we think of it in certain  ways.[15]  For the way in which we think about the world is determined by our understanding of the concepts we use to describe it, and we cannot reliably infer that the world is a certain way from the fact that we conceive of it in that way. To think that one can make such inferences is to confuse  predicates or concepts with  properties,[16]  analytical identities with synthetic identities. We know what we mean by certain concepts by a priori reflection, but the nature of the things to which these concepts refer can only be discovered by empirical investigation. We did not discover that water is H2O or that heat is mean kinetic molecular energy by a priori reflection on what we mean by ‘water’ and ‘heat’, but by empirical investigation. Furthermore, we could not object to the view that heat is mean kinetic energy on the ground that this is not what we mean when we think of something as hot. But the intuitionists seem to object to naturalistic accounts of moral properties in precisely this way.