It is a premise of science, not a finding.

From The Heretic: Who is Thomas Nagel and why are so many of his fellow academics condemning him? by Andrew Ferguson

Materialism, then, is fine as far as it goes. It just doesn’t go as far as materialists want it to. It is a premise of science, not a finding. Scientists do their work by assuming that every phenomenon can be reduced to a material, mechanistic cause and by excluding any possibility of nonmaterial explanations. And the materialist assumption works really, really well—in detecting and quantifying things that have a material or mechanistic explanation. Materialism has allowed us to predict and control what happens in nature with astonishing success. The jaw-dropping edifice of modern science, from space probes to nanosurgery, is the result.

But the success has gone to the materialists’ heads. From a fruitful method, materialism becomes an axiom: If science can’t quantify something, it doesn’t exist, and so the subjective, unquantifiable, immaterial “manifest image” of our mental life is proved to be an illusion.

Here materialism bumps up against itself. Nagel insists that we know some things to exist even if materialism omits or ignores or is oblivious to them. Reductive materialism doesn’t account for the “brute facts” of existence—it doesn’t explain, for example, why the world exists at all, or how life arose from nonlife. Closer to home, it doesn’t plausibly explain the fundamental beliefs we rely on as we go about our everyday business: the truth of our subjective experience, our ability to reason, our capacity to recognize that some acts are virtuous and others aren’t. These failures, Nagel says, aren’t just temporary gaps in our knowledge, waiting to be filled in by new discoveries in science. On its own terms, materialism cannot account for brute facts. Brute facts are irreducible, and materialism, which operates by breaking things down to their physical components, stands useless before them. “There is little or no possibility,” he writes, “that these facts depend on nothing but the laws of physics.”