Argument
In a famous passage Frege wrote:
"If someone wants to say today what he expressed yesterday using the word 'today', he will replace this word with 'yesterday'. Although the thought is the same, the verbal expression must be different in order that the change of sense which would otherwise be effected by the differing times of utterance may be cancelled out."
What Frege says here is compatible with two possible views. On one view, indexicality is not an intrinsic property of certain thoughts, but a property of certain linguistic expressions or, rather, of the relation between these expressions and the thoughts they (contribute to) express: indexical sentences express thoughts only with respect to context, and different indexical expressions have to be used to express the same thought in different contexts. On another view, indexicality is 'essential' (Perry): it is a property of thoughts themselves and not merely of their linguistic expression. According to this view there are indexical thoughts, corresponding to the indexical sentences that express them. Still, it is possible to hold, with Frege, that 'the thought is the same' when you think of a certain day as 'today' and when, the following day, you think of it as 'yesterday'. Although indexical, the thought that is expressed is 'dynamic' and stays the same through the change of context, despite lower-level differences (Evans).
The second view is attractive but it raises the issue of 'cognitive dynamics' (Kaplan): when does an indexical thought become another thought because of a contextual change, and when does it stay the same despite the change? This workshop is devoted to that issue, and to the more general issue of the nature of indexical thought.