 As a consequence, Cantor suggested a crucial distinction—still regarded as “subjective”, i.e., mathematically not precise, by Hilbert (as late as 1904, see van Heijenoort 1967, p. 131)—between totalities that cannot be conceived as a whole (the inconsistent ones) and those which can be regarded as completed (fertige Menge). Roughly, the former is a collection that cannot be an element of other collections, whereas the latter is a small collection, which can be an element of other collections (see also the entry on  the early development of set theory).  This corresponds to the distinction between classes and sets, later made precise and axiomatized in the class-theoretic approach (von Neumann, Bernays, Gödel); it is reminiscent of the Russellian limitation-of-size doctrine (see  3.1  below; Garciadiego 1992).