Modality and description
Arthur Francis Smullyan · Journal of Symbolic Logic · 1948
There are logicians who maintain that modal logic violates Leibniz's principle that if x and y are identical, then y has every property of x. The alleged difficulty is illustrated in the following example due to Quine.1 A. It is logically necessary that 9 is less than 10. B. 9 = the number of the planets. C. Therefore, it is logically necessary that the number of the planets is less than 10.