 That is, if \(A\) is a formula, then \(\osim A\) is false if \(A\) is true, and \(\osim A\) is true if \(A\) is false. The function \(f_{\osim }\) is said to be a truth function because it is a function defined on the set of classical truth values \(\{1, 0\}\), see the entry on  truth values.