Charles Mills has contributed significantly to popularizing the distinction between ideal and non-ideal theory, which has long since ceased to be confined to political philosophy. What has been less frequently noted is that he turned the distinction into a philosophical “demarcation line” in the sense of Louis Althusser. However, those using it tacitly accept the postulate of pure reason and thus squander the opportunity to doing critical rather than merely non-ideal theory. The demarcation line therefore needs to be corrected.