 Theorists tend to be frugal in their postulation of new entities. When a trace is observed in a cloud-chamber, physicists may seek to explain it in terms of the influence of a hitherto unobserved particle. But, if possible, they will postulate one such unobserved particle, not two, or twenty, or 207 of them. This desire to minimize the number of individual new entities postulated is often referred to as quantitative parsimony. (For arguments that the qualitative parsimony / quantitative parsimony distinction is not so clear, see Sendlak 2018.) David Lewis articulates the attitude of many philosophers when he writes: