 Hume’s Principle which is described in Frege (1884: §63) as asserting that the content of the proposition that “the number which belongs to the concept F is the same as the number which belongs to the concept G” is equivalent to “the concept F is similar to the concept G”. In terms of classes this becomes \(\alpha \approx \beta \equiv \# \alpha = \# \beta\). Hume’s principle is the focus of much of the discussion of “Neo-Logicism”, the doctrine that Frege’s construction of the numbers can be built on a consistent foundation (see the entry on   Frege’s theorem).