 Philosophical explorations of the distinction have generated a number of interesting questions in philosophy of language and metaphysics. The linguistic distinction between what are called mass expressions (e.g., ‘lead’, ‘the water in the tub’) and count expressions (e.g., ‘two horses,’ ‘the horse in the barn’) is believed to be important in examining the metaphysical stuff/thing distinction. In this entry we will be more concerned with nominal mass expressions (e.g., ‘the water in the tub’, ‘the gold in his teeth’), as opposed to predicative mass expressions (e.g., ‘the statue is lead’). While there are many issues in this area, we will be focused on the following kind of questions: Are things illusory, the world consisting, most fundamentally, of stuff alone? Or is stuff just a certain kind of thing or things? Is all apparent talk about physical kinds of stuff really veiled talk about individual concrete portions of matter, or is matter per se an irreducible part of the furniture of the world?