Unger argues that some philosophical problems have no solution because certain key terms can be understood in two different ways. Since neither way of understanding these terms is better than the other, there is no correct solution to the problems in which these terms play a role.  

He offers the word “flat” as a typical, non-philosophical term that has two such senses. In one sense, something is flat if it is relatively flat compared to other things of a similar or different nature, depending on the context. Kansas is flat compared to Vermont, and the tops of coffee tables are flat compared to lots of other objects. Unger calls this the “contextualist” case.  

In another sense, however, Kansas clearly isn’t perfectly or absolutely flat, nor are coffee tables. The only thing that is flat in this sense is probably a plane as defined in geometry. Unger calls this the “invariantist” case, since the meaning of the specified term in this case doesn’t vary from context to context.  

Unger identifies four philosophical problems that he thinks are subject to this kind of ambiguity: the problems of knowledge (the word “know”), free will (words like “can” and “could”), causation (“cause”) and explanation (“explain”).  

For example, we commonly say that we know many things, but, when pressed, we confess that we could be wrong. Which standards must be met for knowledge to exist? Should our everyday standards be applied (“I saw a dog in the car going by”) or much more stringent standards that would rule out any possibility of error (“I  stopped the car and confirmed that a dog was present by sight, touch and hearing; discussed the matter with other observers; and then performed a series of medical tests to verify that the dog was a living organism with canine DNA”)?  

As Unger points out, we could still be wrong relative to the very highest standards, except possibly with knowledge of the “Cogito, ergo sum” variety (“I know that there is something”.) Understanding terms in the invariantist fashion can obviously lead to skepticism, but Unger argues that skepticism may be warranted — there is no right answer when it comes to choosing between contextualist and invariantist positions. He offers extended discussion of semantics vs. pragmatics and semantic intuitions, but his basic point is that some important terms are ambiguous and there are no compelling reasons to choose one meaning over another. 

There are at least three different kinds of relativity involved here. First, there is the idea that the meanings of some terms are relative to the context in which they are used (this is the position called “contextualism”). Second, there is semantic relativity: the idea that the meaning of certain terms is relative to certain assumptions, e.g. the standards that are appropriate for saying that someone knows something. Third, there is philosophical relativity: the idea that some semantically relative terms are philosophically significant, and that this semantic relativity results in certain philosophical problems having no solution. Unger argues in favor of all three kinds of relativity. 

It seems quite correct to say that some terms are contextually and semantically relative, and that some of these terms play a key role in philosophical disputes. I’m not sure that this explains why these disputes are hard to solve, however. For example, it seems clearly true that if we understand “know” in the ordinary sense, we know many things, and if we understand “know” in the ideal sense, we don’t know much at all. Unger doesn’t spend much time explaining why this ambiguity is so crucial. Few philosophers would deny that this kind of ambiguity exists, yet they would continue to argue about the nature of knowledge and justification, and whether or not there are grounds for choosing between the ambiguous meanings Unger describes.  (5/16/12)