I was reading Peter van Inwagen's introduction to metaphysics recently, and he uses an argument against Anti-Realism which I've seen all too often and which I think badly misrepresents the Anti-Realist. I've seen the same sort of argument used against the Sceptic. First, let's just throw some definitions out there:
Global Scepticism (GS): The proposition that knowledge about any proposition is impossible.
Global Anti-Realism (GAR): The proposition that there is no objective reality, and therefore no true or false propositions.
The van Inwagens of this world use the following sort of argument:
GS is self-contradictory because if you believe that there is no knowledge, then how can you know that? It's simply self-contradictory to assert that all propositions are unknown because to assert that statement is to assert that that statement is known, and that statement is a member of the set of all propositions. Therefore even asserting Global Scepticism is as meaningless to your audience as asserting "This statement is false".
Likewise with GAR. If you argue that no propositions have the property of being true, you automatically assert that that statement is true.
Now in the cases of both GS and GAR there is one consistent position that van Inwagen may allow. Call these WGS and WGAR respectively ("W" for "Weak"). WGS states that "The only known proposition is that no other propositions are known." This is a lot like the famous saying of Socrates which went something like, "The only true wisdom is in knowing you know nothing", but don't confuse the two as the latter is a kind of aphorism, whereas the former is truly believed by an advocate of WGS. It could be argued that WGS is also contradictory because once you know that you can't know anything, you may also know a few more things, like "Either nothing is known, or nothing is known." But I disagree. The whole point of Global Scepticism is to deny knowledge of everything, including that there are axioms of logic which entail that "A or A" follows from "A". I think WGS and WGAR are consistent positions, but they are a little arbitrary.
van Inwagen argues that a GARist is forced to express something like "It seems to me that there is no objective truth", which is not incompatible with "It seems to you that there is an objective truth.", and that therefore the GARists are forced to just shut up and let the realists carry on with their lives.
I believe that both GS and GAR are consistent propositions, and this is why. They should not be expressed as I have expressed them in my original definitions, which are quite similar to how most people define them. Instead they should be defined like this:
Global Scepticism: If knowledge is defined as XYZ, then it is not the case that knowledge exists. (Or there exists no proposition which has the property of being known).
Global Anti-Realism: If objective truth is defined as ABC, then there is no proposition with the property of truth.
These sentences are not self-contradictory, because they do not self-refer. These positions actually assert that the opposing views (let's call them Realism and Epistemic Realism) are false, at least given the definitions of truth and knowledge available and agreed upon. Global Sceptics are not asserting that they know that nothing is knowable. Instead they are asserting that if we adopt a certain definition of knowledge, then nothing in the world corresponds to that definition.
This position is sometimes referred to as Pyrrhonism, and distinguished from regular Scepticism. Why it is distinguished I have no idea, as it's immediately obvious that Scepticism, understood as how I defined Global Scepticism at the beginning of this post, is self-contradictory, whereas Pyrrhonism is quite sensible. I know of no similar distinction with regard to Anti-Realism though. All the GARist is really pointing out is that you cannot reasonably accept some definitions of "truth" and also believe that anything is true, because actually nothing has the property that you define as truth. That is all that is going on. I think that most Anti-Realists are merely saying that truth is an impossible thing, and what is important is some other thing that they may try to analyse. Likewise most Sceptics argue that knowledge, defined as justified true belief, does not exist, but that there is some other epistemic notion which is important and worth looking for and worth having.
