philosopher makes mistake

In “How to Define Theoretical Terms” (1970), David Lewis says the following. Take a theory T that introduced a new term ‘t’. Replace ‘t’ in T with an appropriate variable to form an open sentence R`. Lewis now claims that ‘t’ is correctly defined as follow:

t = the unique x such that R`

Note the uniqueness requirement. If there are multiple realizations of R` (that is, variable assignments that satisfy R`) differing in what they assign to x, then ‘t’ is denotationless. Van Fraassen (1997) argues that, provided only that T is consistent and has an infinite model, such will always be the case.

The Road to Ramseyan Humility

I’ve mentioned to a few of you around here that I think it is Lewis’ solution to “Putnam’s Paradox” that lands him in “Ramseyan Humility”. Here’s a sketch of why. I apologize for assuming some background familiarity with both “Putnam’s Paradox” and “Ramseyan Humility”, but I’m trying to keep things punchy.

Consider Lewis’ argument for Humility. Lewis’ assumes that T is realized by fundamental properties, which (because fundamental properties are plausibly wholly distinct from the roles they realize) allows Lewis to run the permutation argument to establish multiple possible realizations of T. But why does Lewis assume that T is realized by fundamental properties? There are actually several questions here. First, why does Lewis think that T is even prima face the sort of thing that is realized—that is, why does Lewis think that the Ramsey sentence approach to the content of T is the right one? Second, why does Lewis think that T is realized (rather than holding a non-representational interpretation of the Ramsey sentence as Sklar does)? Third, assuming that T is realized by something, why does Lewis think that T is realized by (metaphysically robust) natural properties [rather than, say, (metaphysically thin) classes]? And, finally, fourth, assuming that T is realized by natural properties, why does Lewis think that T is realized by completely natural properties, i.e., fundamental properties (rather than, say, natural “enough” properties).

Although there are four questions here, I think Lewis’ answer to the first three will be roughly the same: realism. His answer to the fourth question, however, can be found in RH where he makes a particular observation about scientific progress. Let’s take these questions in order.