Editing Propositional calculus (section)

=== Constants and schemata ===
Mathematicians sometimes distinguish between propositional constants, [[propositional variable]]s, and schemata. ''Propositional constants'' represent some particular proposition,<ref name=":9"/> while ''propositional variables'' range over the set of all atomic propositions.<ref name=":9" /> Schemata, or ''schematic letters'', however, range over all formulas.<ref name="BostockIntermediate"/><ref name=":1" /> (Schematic letters are also called ''metavariables''.)<ref name=":35" /> It is common to represent propositional constants by {{mvar|A}}, {{mvar|B}}, and {{mvar|C}}, propositional variables by {{mvar|P}}, {{mvar|Q}}, and {{mvar|R}}, and schematic letters are often Greek letters, most often {{mvar|φ}}, {{mvar|ψ}}, and {{mvar|χ}}.<ref name="BostockIntermediate" /><ref name=":1" />

However, some authors recognize only two "propositional constants" in their formal system: the special symbol <math>\top</math>, called "truth", which always evaluates to ''True'', and the special symbol <math>\bot</math>, called "falsity", which always evaluates to ''False''.<ref name="ms18"/><ref name="ms19"/><ref name="ms20"/> Other authors also include these symbols, with the same meaning, but consider them to be "zero-place truth-functors",<ref name="BostockIntermediate" /> or equivalently, "[[nullary]] connectives".<ref name=":33" />