31.3. Ramseyan Humility

Rae Langton (1998) has offered a new reading of Kant’s thesis that we have no knowledge of things in themselves. According to her, the thesis does not mean there is a class of things behind the scenes about which we know utterly nothing; rather, it means we have no knowledge of the intrinsic properties of the things we encounter every day—no knowledge of how they are “in themselves,” though we may have knowledge of their relational properties.This is the thesis she calls Kantian Humility.

In the statement “we have no knowledge of the intrinsic properties of bodies” there is an apparent echo of Hume, but there are two differences worth noting. First, in one way Lewis’s thesis is more modest. As noted above, the skepticism affirmed by Hume is conceptual skep­ticism—we cannot even form a good conception of what bodies might be. The skepticism affirmed by Lewis is epistemic skepticism—he allows that we can conceive of various intrinsic properties of bodies, but argues that we cannot know which of these properties are actually instantiated by bodies. Second, in another way Lewis’s thesis is more radical. Hume allows that we can know the intrinsic properties of some things—for example, the colors and shapes of our own impressions. But Lewis maintains that “humility spreads,” making us ignorant even of the intrinsic properties of our own qualia.⁷

Lewis's argument for Ramseyan Humility consists of three premises—the Ramsey premise, Combinatorialism, and quidditism.

To state the first premise, we must briefly review Ramsey's method for dealing with theoreti­cal terms, which I shall do with a made-up example of a simple theory for explaining the tastes of apple and lemon juice.The theory may be written like this:

Lemon juice contains negative particles, and whatever contains negative particles is sour; apple juice contains positive particles, and whatever contains positive particles is sweet.

The theory contains observational terms, such as “lemon juice” and “sour,” which we under­stand because we are acquainted with the things or properties they stand for. It also contains theoretical terms, such as “positive particle,” which stand for things or properties we are not acquainted with. How, then, are we to understand the theory? Ramsey proposed that we replace all the theoretical terms by variables and then write out the existential generalization of the resulting formula with respect to every variable, as follows:

3F3G(Lemon juice contains Fs, and whatever contains Fs is sour; apple juice contains Gs, and whatever contains Gs is sweet).

Gone now are the theoretical terms. We can understand the resulting formula—the Ramsey sentence for the theory—if we understand the observational terms plus logic.⁸

Now for Lewis's Ramsey premise. It is provable that a theory and its Ramsey sentence have all the same observational consequences, so two theories with the same Ramsey sentence have all the same observational consequences. Lewis assumes that any evidence we have for a theory must consist in its record of predictive success—in its observational consequences that come out true; hence that no amount of observation will tell us whether a given theory or another theory with the same Ramsey sentence is true; hence that we could never know which is true.⁹ This is the epistemological premise in his argument, which combines with the other two premises to yield Humility as the conclusion.¹⁰

The second premise in Lewis's argument is combinatorialism.This is the principle that if the elements (including the properties and relations) in any possible situation are taken apart and rearranged (for example, one n-place relation switched with another), the result is again a pos­sible situation. As applied to our toy theory, it implies that if the original was possible, so is the proposition expressed by

Lemon juice contains positive particles, and whatever contains positive particles is sour; apple juice contains negative particles, and whatever contains negative particles is sweet.

To get from the original to the above, we have simply permuted the terms “positive particle” and “negative particle”—or, as Lewis also says, permuted the properties themselves in the realization of the original theory.¹¹ As Lewis notes, combinatorialism implies that the laws of nature are contingent. If it were necessary that positive particles make for sweetness, our permuted theory would not express a possibility.

The third premise is quidditism. Quidditism tells us that the result of permuting properties is not only a possibility, but a possibility distinct from the one we started with. “Two different possibilities can differ just by a permutation of fundamental properties” (209).The import of this will be better appreciated when we consider below structuralist views that deny it. It can also be appreciated by its analogy with haecceitism—the view that you can permute the individuals in a situation and get a distinct possibility.¹²

We can now see how the three premises come together to yield Ramseyan Humility. By combinatorialism, the properties of being positive and negative could change places. By quid- ditism, if they did, the result would be a possibility distinct from what we started with. And by the Ramsey premise, we could never know that one rather than the other of these possibilities was actual.This is one of countless cases in which we could not know which intrinsic properties are possessed by the particles composing this or that stuff.

How Humean are Lewis's premises? Lewis presents the combinatorial premise as a develop­ment of Hume's dictum that there can be no necessary connections among distinct existences. As for quidditism, Hume would agree if the permuted properties were observable properties, but in Lewis's argument they are theoretical properties, in which case the question may not arise for Hume. Insofar as the properties are theoretical, Hume would say they are properties of which we have no idea because no relevant impressions.What, finally, of the Ramsey premise? It is in the spirit of Hume's empiricism to say that if two theories have all the same observational con­sequences, we cannot know that one rather than the other is true; but that may only be because Hume would not allow any content to a theory over and above its observational consequences.

The main point of affinity between Lewis and Hume is that they both make assumptions that would be challenged by proponents of structuralism, to be examined in the next section.

31.4