 The sortals that Wiggins regards as crucial, the substance sortals, are a proper subset of the ones Strawson is concerned with. Substance sortals contrast with phase sortals; the latter typically only apply to some temporal segment of an object. Kitten is a phase sortal because when a cat matures it ceases to be a kitten but it does not go out of existence. Substance sortals are also contrasted with restricted sortals, e.g., “red table”. Wiggins has a detailed theory of the structure of substance sortals, arguing that if any two substance sortals overlap, then either one is a restriction of the other or both are restrictions of some other sortal.