  While it is generally agreed that signs for negation, conjunction, disjunction, conditionality, and the first-order quantifiers should count as logical constants, and that words like “red”, “boy”, “taller”, and “Clinton” should not, there is a vast disputed middle ground. Is the sign for identity a logical constant? Are tense and modal operators logical constants? What about “true”, the epsilon of set-theoretic membership, the sign for mereological parthood, the second-order quantifiers, or the quantifier “there are infinitely many”? Is there a distinctive logic of agency, or of knowledge? In these border areas our intuitions from paradigm cases fail us; we need something more principled.