Back after a software-imposed break. While all this was going on, Maverick philosopher commented on my Frege post, gently supportive but questioning whether cognitive value or informational content can be had by such subsentential items as 'morning star' and 'evening star.' I think it can be had. We both agree the proposition "grass is green" has informational content. This content differs from the content of "grass is yellow". How? How does changing the word 'green' to the word 'yellow' change the informational content of the whole proposition? Surely because the informational value of the word 'green' is different from the informational value of the word 'yellow'. Thus the informational value of a term is precisely its contribution to the informational value of the whole sentence. Frege says somewhere that the same sense corresponds to the same word, and that is why language is useful. We could use a single sign for the whole proposition, as with a train signal (red = the track is closed, green = the track is clear), but he says that would not be very helpful, as we would have to devise a new sign for every different thought we wanted to express. So we split the thought up into parts, with a different sign corresponding to each part of the thought. If we understand 'informational value' in this way - as the contribution to the informational value of a whole proposition, my argument is sound.
Cmmenting on the post back here, Belette wondered whether my argument was valid at all. I argued as follows.
(1) The sentence “the morning star is the evening star” has informational content.
(2) The sentence “the morning star is the morning star” does not have informational content.
(3) Therefore, the term “the morning star” does not have the same informational content as “the evening star”.
Belette says that ‘the morning star’ really means ‘Venus, at certain positions in its orbit’. I’ll alter this (because I’m not sure the morning/evening distinction is anything to do with the orbit of Venus, as opposed to the rotation of the earth on its axis) to ‘Venus as it is in the morning’. Perhaps I am not understanding him, but I think he means that this expression is something that has a referent in the morning, but at no other time. As the earth rotates and Venus appears to rise in the sky and disappear in the glow of the rising sun, the reference of the expression fails, and becomes non-existent. And as the sun ‘sets’, the referent of ‘Venus as it is in the evening’ comes into existence. Thus the referent of ‘Venus as it is in the morning’ only exists in the morning, and the referent of ‘Venus as it is in the evening’ only exists in the evening. Venus, of course, exists all the time.
But if that is true, the argument above is no longer sound. The sentence “the morning star is the evening star” actually means “Venus as it is in the morning is Venus as it is in the evening”. But this is always false, for if the term on the left has a referent, it must be morning, in which case the term on the right has no referent, and the identity statement is false. Conversely, in the evening, the left term has no referent, and the right term does. Or neither term has a referent, in which case there can be no identity either (identity being a relation between existing things). Hence the second premiss is false, on the assumption that a sentence which is false in virtue of its meaning contains no information.
I question whether ‘the morning star’ does mean that. Surely it is a description picking out a certain object by means of its visibility in the morning. It is not part of its meaning that the object must cease to exist once morning has passed. It may cease to exist, of course, just as the light in the fridge ceases to exist when we shut the door. But continuation or cessation of existence is not part of its meaning. (Perhaps it is different for ‘the prime minister of England’, but that is a different case, I think).