 Another feature widely believed to be defining of logic is its formality: the truth of a principle of logic is guaranteed by the form of thought and/or language and does not in any way depend upon its matter. What this feature amounts to will obviously depend on how the distinction between form and matter is understood. The most popular explication of the distinction between form and matter derives from the widely shared view that no objects exist by conceptual necessity (Field 1993; Yablo 2000). On this view it is natural to regard anything having to do with the existence of objects and with their particular characteristics as belonging to the matter of thought rather than to its form. This gives rise to two features that are often regarded as defining of logic. Firstly, logic has to be ontologically innocent; that is, a principle of logic cannot introduce any new ontological commitments (Boolos 1997; Field 1984). Secondly, the basic notions of logic must not discriminate between different objects but must treat them all alike. This latter idea is often spelled out as the requirement that logical notions must be invariant under permutations of the domain of objects (Tarski 1986).