The expressions selected to indicate the logical form of a sentence. A sentence will play a proper role in inference in virtue of a structure that can be exhibited by means of these expressions. The basic logical constants include expressions for the truth functions &, ∨, →, ↔, and ¬, the quantifiers (∃) and (∀), and the identity relation =. The parentheses, ( ), which indicate the scope of functions, may be counted as logical constants. Richer conceptions of logical form will include other constants: modal logic includes the operators □ and ◊, and tense logic will include operators signifying past, present and future. See Appendix 1: Logical Symbols.