"First, there is what is called the materialist view."

A "materialist" may be simply a person inordinately consumed with "things"--money, cars, possessions, and so on. Here, though, Lewis means more than that, of course. "Materialism . . . [is] the theory that physical matter is all there is. This theory . . . states that everything in the universe is matter, without any true spiritual or intellectual existence" (allaboutphilosophy.org).