Is all morality subjective?

How do we know that cruelty is wrong? We cannot point a camera at "wrongness," nor can we isolate it in a particle accelerator. If objective moral values exist, our access to them seems to require a cognitive pathway that defies the boundaries of natural science. This is the core of J.L. Mackie's epistemological challenge in [*Ethics: Inventing Right and Wrong*](https://en.wikipedia.org/wiki/Ethics:_Inventing_Right_and_Wrong). To mount the strongest possible defense of Mackie's skepticism, we must not treat "queerness" as a mere dismissal. Instead, we must ground it in a sophisticated epistemological framework: if moral properties are causally inert, any claim to know them violates our best scientific understandings of how human minds acquire knowledge. ## The Causal Imperative of Knowledge The strongest formulation of Mackie’s epistemological challenge rests on the **causal theory of knowledge**. In contemporary epistemology, as formulated by thinkers like [Alvin Goldman](https://plato.stanford.edu/entries/knowledge-analysis/#CausAllo), for a belief about the external world to count as knowledge, there must be a systematic causal connection between the belief and the fact that makes it true. When we perceive a physical object, photons bounce off the object and stimulate our retinas. But moral facts—such as "gratuitous cruelty is bad"—are non-physical and causally inert. They cannot emit photons or exert gravitational pull. Therefore, a defender of Mackie can argue: 1. To know an objective external fact, that fact must causally impact our cognitive faculties. 2. Objective moral values, being non-natural and non-physical, cannot exert causal power. 3. Therefore, we cannot possess a natural faculty for detecting them. As the philosopher [Gilbert Harman](https://en.wikipedia.org/wiki/Gilbert_Harman) famously argued in *The Nature of Morality*, we do not need to posit objective moral facts to explain our moral observations; our psychological conditioning and evolutionary history are entirely sufficient. ## The Failure of the Mathematical Analogy Moral realists, such as [rational intuitionists](https://plato.stanford.edu/entries/intuitionism-ethics/), often attempt to rescue moral knowledge by comparing it to mathematics. They argue that we grasp mathematical truths (like $2 + 2 = 4$) without sensory interaction, so why not moral truths? However, a sophisticated steel-man of Mackie's objection dismantles this parallel. Mathematical truths are analytical, conceptually necessary, or indispensable for our best physical sciences. Moral realism enjoys no such status. The philosopher Hartry Field developed a formidable objection to mathematical realism—later adapted for ethics by [Sharon Street](https://en.wikipedia.org/wiki/Sharon_Street) as the "Darwinian Dilemma"—which argues that our evaluative beliefs are thoroughly shaped by evolutionary pressures. > "Allowing that forces of natural selection have heavily influenced our evaluative attitudes... the realist must claim that there is an amazing coincidence between the independent moral truths and the evaluative attitudes evolution thrust upon us." > — Sharon Street, [*A Darwinian Dilemma for Realist Theories of Value*](https://doi.org/10.1086/505234) If our moral intuitions are merely survival-promoting heuristics selected by evolution, the claim that they also happen to align perfectly with independent, objective moral facts is a cosmic coincidence of highly improbable proportions. ## The Unavoidable Appeal to "Magic" Ultimately, the steel-man of epistemological queerness forces the realist to posit a faculty of "intuition" that lacks any neurobiological or evolutionary plausibility. By demonstrating that moral knowledge cannot be integrated into a naturalistic worldview, Mackie’s skepticism remains a formidable challenge: to reject it, one must reject the very scientific framework that explains all other human knowledge.