Hylas proposes a material substratum, which underlies sensible qualities (color, heat, shape, extension, etc.) "without which they [the sensible qualities] cannot be conceived to exist." (P 168b)

The substratum is not itself sensible. (P 168b)

"The word `substratum' is used only to express in general the same thing with `substance.'" (P 169a)

Berkeley argues that there is not a material substratum, or rather that that we have "no idea at all, neither relative nor positive," of what this would be. (P 169a)