Logicians think (or used to think) that, if two phrases denote the same object, a proposition containing the one may always be replaced by a proposition containing the other without ceasing to be true, if it was true, or false, if it was false. But, as we have just seen, you may turn a true proposition into a false one by substituting 'Scott' for 'the author of Waverley'. This shows that it is necessary to distinguish between a name and a description: 'Scott' is a name, but 'the author of Waverley' is a description.
Source: My Philosophical Development, chap. 7：1959．
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