 Are these sufficient grounds for rejecting empiricism as a methodology? If moral concepts were alone in presenting a difficultly for empiricism, then there might be grounds for maintaining that epistemology, and either denying that there are moral concepts, or at least denying that we have moral knowledge. Price’s main argument against empiricism about concept acquisition consists in finding what Mackie later called companions in guilt; i.e., other concepts which are indispensable, and which we happily use, that cannot be explained on empiricist epistemology of this sort. Price’s rather familiar examples include: solidity, substance, power, cause, duration, inertia, infinity, number, identity, equality, and all modal concepts. If empiricists are successfully to defend their methodology they must either show that the concepts in question can be explained on their epistemology, or bite the bullet and deny that we do have any clear understanding of concepts like these or, at least, that our ideas about such matters are confused or misleading. For Price, by contrast, it is the understanding (a branch of reason), and not sense experience, that is the source of such concepts.