Frege’s next really significant work was his second book, Die Grundlagen der Arithmetik: eine logisch mathematische Untersuchung über den Begriff der Zahl, published in 1884. Frege begins this work with criticisms of previous attempts to define the concept of number, and then offers his own analysis. The Grundlagen contains a variety of insights still discussed today, such as: (a) the claim that a statement of number (e.g., ‘there are eight planets’) is a higher-order assertion about a concept (see Section 2.5 below); (b) his famous Context Principle (“never ask for the meaning of a word in isolation, but only in the context of a proposition”), and (c) the formulation of a principle (now called ‘Hume’s Principle’ in the secondary literature) that asserts the equivalence of the claim “the number of Fs is equal to the number of Gs” with the claim that “there is a one-to-one correspondence between the objects falling under F and the objects falling under G” (see Section 2.5 below). More generally, Frege provides in the Grundlagen a non-technical philosophical justification and outline of the ideas that he was to develop technically in his two-volume work Grundgesetze der Arithmetik (1893/1903).