Part III treats systematic challenges to natural perfectionism, and opens with the so-called fact/value dichotomy. This challenge can be parsed in four main ways. First, the metaphysical challenge, which has historical roots in Hobbes and Hume. This holds that the ‘natural’ cannot accommodate the normative: a claim I argue is question-begging, depriving norms, furthermore, of any proper grounds. Second, the inferential challenge, which maintains that one cannot move validly from ‘is’-type propositions to ‘ought’-type ones. This Humean challenge fails, I argue, since natural perfectionism rests its claims on natural facts that are already inextricably inflected with value. Third, G. E. Moore’s semantic challenge. Moore claims that any naturalistic definition of ‘good’ both commits the ‘naturalistic fallacy’ and falls foul of the ‘open question argument’. I argue that the former is a pseudo-fallacy and that the latter conflates not seeming ‘closed’ with being ‘open’. Fourth, the conceptual challenge attacks ‘thick’ concepts, these being purportedly inextricable amalgams of ‘fact’ and ‘value’. I argue that thick concepts are defensible, for pragmatic, grounding and moral reasons.