Saturday, 22 December 2007

Frege's Venus, Sense and Reference

I've never studied logic formally at university, but I have read up on the subject here and there. What I always found most confusing, however, was the presentation of the classic example illustrating the difference between sense and reference, that of the morning star and the evening star. The problem was that in quite a few noteworthy books, the point of the example was completely lost on me because it was not explicitly explained that the morning star and the evening star are actually the same object!

Thankfully though, I did discover what the morning star and evening star were referring to and, consequently, why distinguishing between sense and reference is so important.

And that is also why I'm so happy to have read perhaps the most lucid explanation of this distinction in AC Grayling's An Introduction to Philosophical Logic, and is why I'm hoping all books on logic will just reprint the following passage from here on in:
The classic example used is the planet Venus, which the Greeks thought was not one planet but two stars, namely the evening star Hesperus and the morning star Phosphorus. Because the evening and morning stars are the same entity, it is evident that both expressions denote the same entity, viz., Venus. But clearly the expressions 'morning star' and 'evening star' differ in sense despite having the same reference, which follows from the fact that if one says 'the morning star is identical with the morning star', the truth of what one says is a simple matter of logic and can be determined by inspecting the sentence itself; but if one says 'the morning star is identical with the evening star', the truth of what one says is a matter of astronomy, not logic. No one could discover that the morning and evening stars are in fact one and the same entity merely by inspecting the expressions 'the morning star' and 'the evening star' alone. It follows that although these two expressions are coreferential, which is to say, refer to the same thing, they differ in sense

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