 Since the introduction of the modern predicate calculus, it has been common to use three separate logical notations (“Px”, “x = y”, and “∃x”) to represent three separate senses of the natural-language word “is”: the is of predication, e.g. “Cicero is wise”; the is of identity, e.g. “Cicero is Tully”; and the is of existence, e.g. “Cicero is”. It was Russell’s suggestion that, just as we use logic to make clear these distinctions, we can also use logic to discover other ontologically significant distinctions, distinctions that should be reflected in the analysis we give of each sentence’s correct logical form. 