The two central problems concerning "logical terms" are demarcation and interpretation. The search for a demarcation of logical terms goes back to the founders of modern logic, and within the classical tradition a partial solution, restricted to logical connectives, was established early on. The characteristic feature of logical connectives, according to this solution, is truth-functionality, and the totality of truth functions (Boolean functions from n-tuples of truth values to a truth value) determines the totality of logical connectives. In his seminal 1936 paper, "On the Concept of Logical Consequence," Alfred Tarski demonstrated the need for a more comprehensive criterion by showing that his semantic definition of logical consequence—the sentence σ is a logical consequence of the set of sentences Σ iff (if and only if) every model of Σ is a model of σ—is dependent on such a demarcation. (Thus suppose the existential quantifier is not a logical term...