Darwin's theory of evolution.
William Ockham (or Occam) was a thirteenth-century scholastic philosopher. His principle gives preference to simplicity in cases where there are competing explanatory theories.
Materialists often cite Ockham’s Razor in support of their position. Ockham’s Razor is the argument that in any given argument, the position with the most metaphysical simplicity is the more likely position. At its core, it means that the simpler the position, the more likely it is to be true. Materialists deploy this argument as evidence for materialism’s superiority over dualism. Because materialism rests on the existence of just one world, the physical one, rather than hinging on the existence of two components, it is the simpler argument. Both dualism and materialism are unproven theories. But according to Ockham’s Razor materialism is the more likely explanation due to its simplicity.
Ockham's Razor does not mean materialism is true. It simply states that in cases where there are competing claims, the simpler claim is more likely to be true. In science and history, the more complex explanation has frequently been proven true. Alfred North Whitehead summed up the dangers of relying heavily on Ockham's Razor as evidence for one argument over another. He said, "the aim of science is to seek the simplest explanation of complex facts. We are apt to fall into the error of thinking that the facts are simple because simplicity is the goal of our quest. The guiding motto in the life of every natural philosopher should be “Seek simplicity and distrust it.”
The simplest explanation for something is usually the correct one. Asserting humans are made of one substance instead of two is the simpler explanation. Therefore, it is more likely that we are made of one substance not two.
That doesn't mean materialism is true.