Objective Reality and Invariance

The great lesson of modern physics is that objective properties are invariant properties — those that do not depend on the particular standpoint from which they are observed. Special relativity showed that the speed of light is invariant across all inertial reference frames; general relativity expressed the laws of physics in a form invariant across all coordinate systems. Nozick generalises this criterion to philosophy: something is objectively real — as opposed to merely subjective, perspective-dependent, or conventional — to the degree that it remains invariant across the relevant class of transformations. This gives us a way to assess objectivity in ethics, mathematics, and consciousness, not just physics.

Nozick applies the invariance criterion to moral claims. Some moral intuitions — prohibitions on gratuitous cruelty, the wrongness of betrayal, the importance of fairness — display remarkable cross-cultural and historical invariance. This convergence is evidence of moral objectivity: the intuitions are tracking something real about social cooperation and human flourishing, not merely expressing parochial preferences. The evolutionary origins of moral intuitions do not undermine this objectivity; evolution tracks facts about what kinds of cooperation promote survival and flourishing, and those facts are objective features of social reality, invariant across the relevant range of human situations.

The most challenging case for the invariance criterion is consciousness. Subjective experience — the redness of red, the painfulness of pain — is by definition perspective-dependent and resistant to the kind of third-person description that captures invariant physical properties. Nozick does not claim that consciousness is objective in the full physical sense; he argues instead that the existence of consciousness is itself an objective fact, invariant in the sense that it is acknowledged across all perspectives and cultural frameworks. The challenge is to understand how objective physical processes give rise to subjective experience — a challenge Nozick treats as real and unresolved, not to be dissolved by identifying consciousness with its functional or computational correlates.

Invariances (2001) was Nozick's last book, published five years before his death in 2002. Its synthesis of philosophy of science, metaphysics, and ethics represents the culmination of the exploratory philosophical style he had practised since Philosophical Explanations. The invariance criterion has been compared to Quine's naturalism and to structural realism in philosophy of science.