 As mentioned above, Bolzano defines an idea as a part of a proposition, more precisely, as “any constituent of a proposition that is not itself a proposition” (TS, §128, no. 2 [II.18]; cf. §48). For example, assuming the sentence ‘Fido, who is a dog, is not a reptile’ to be a fairly distinct expression of the corresponding proposition, we might distinguish within that proposition ideas (i.e., sub-propositional parts) such as [Fido], [dog], [reptile], but also [who], [is], and [not]. As with propositions, Bolzano carefully distinguishes between ideas in themselves, thought (or subjective) ideas, and linguistic signs for ideas.