Can a proposition change its truth value?

Question

In Anthony Kenny's a New History of Philosophy he says

According to a modern conception of the nature of the proposition, no proposition can be at one time true and another time false

However my understanding of propositions is that some of them can change their true value. For instance:

The rain in Spain falls mainly on the plain

is true (for the sake of argument) at all times. However

Today it is raining

might be true on the 18th May 2014 but false the day afterwards. So it's truth value can change.

So what have I done wrong? Have a misunderstood the nature of a proposition or truth value? Or is there more than one 'modern conceptions' of a proposition?

As ever, many thanks for everyone's expertise.

Answer 1

There are two main views on the semantics of propositions: tensed vs tenseless. Quoting Markosian:

Tensed view: Propositions have truth values at times rather than just having truth values simpliciter. [This makes it] possible for a proposition to have different truth values at different times.

Tenseless view: Propositions have truth values simpliciter rather than having truth values at times. [This makes it impossible] for a proposition to have different truth values at different times.

The 'modern conception' that Kenny is referring to is the one inherited from mathematics, where sentences don't include such locutions as 'I', 'now', 'yesterday' and so on, and thus the need to make sentences temporally unstable (to use Rescher's phrase) doesn't arise.

The tenseless view of propositions is still prevalent in the mathematical practice, but it seems a little too strong to claim that it's the modern conception of the semantics of propositions. The alternatives are out there, and philosophers of time, logicians interested in the temporal modalities, and others in linguistics and computer science departments are debating the pros and cons of each view. So yes, there is more than one modern conception of the semantics of propositions.

Further Reading. I would recommend reading the aforementioned SEP article on Time in its entirety. If you're interested in a more formal treatment you can look at van Benthem's Modal Logic for Open Minds, chapter 18, and/or Goldblatt's Logics of Time and Computation, chapter 6, both from CSLI.

Answer 2

Just to add some more relevant literature on the debate between eternalists (propostitions don't change their truth-values) and temporalists (eternalism's negation), which at its heart is a metaphysical rather than a formal discussion: Proponents of temporalism include:

• Arthur Prior: Past, Present and Future. Oxford 1967.
• Prior: Papers on Time and Tense. Oxford 1968.
• David Kaplan: "Demonstratives". In Themes from Kaplan ed. Almog et al.. New York 1989.
• Francois Recanati: Perspectival Thought. New York 2007.
• Berit Brogaard: Transient Truths. New York 2012.

Since eternalism has been the orthodoxy until recently there are only few comprehensive defenses of that position. But you may read:

• Gottlob Frege: "Der Gedanke" 1918. A translation is available here: http://philo.ruc.edu.cn/logic/reading/Frege_The%20Thought.pdf
• Mark Richard: "Temporalism and Eternalism". Phil. Studies 39 (1981).
• Richard: "Tense, Propositions and Meanings". Phil. Studies 41 (1982).

Answer 3

Truth functional temporalism, the idea that a proposition's truth value is affected by its relative position in a compound sequence even if no overt tense differentiation is made, can be invoked to resolve certain paradoxes. The barber who decides to shave today all and only those who didn't shave themselves yesterday will shave himself today if he didn't yesterday either, but won't if he did. End of problem. (Ditto mutatis mutandi for the Russell set paradox.)