 This distinction between non-logical, “referring” expressions and logical constants allows us to define a logical truth in a way that has become common (and will be particularly useful in this entry) as a sentence that is true no matter what non-logical expressions occur in it (cf. Tarski, 1936 [1983], Quine, 1956 [1976], Davidson 1980). Consequently (placing non-logical expressions in bold, and re-numbering prior examples):