Oh but I think a relativist can agree that there is a physical world out there in which we all live. And when a relativist agrees with that he'll probably also agree that our sciences have created an impressive collection of rules that offer great predictive behavior (or have such a history). In fact, the relativism which I have come across is extremely dull in that regard. It only happens to be some dogmas and also many people seem to not understand exactly what the consequences are from dropping those dogmas.
What on the other hand, does need to be dismissed are the idea of 1:1 correspondence to reality and the thought that there is something like the scientific method that makes our body of knowledge converge to reality. Fortunately this is, as I said before, not a big deal.
The intuitive idea 1:1 correspondence to reality says that we have knowledge (truth statements) which describes reality at its very essence. By means of an example I'll explain both the relativist's and the essensialist's view on some truth statements. It's not historically accurate but I'll get to that later.
The ancient aspiring physicist might have said “the apple falls from the tree.” True says the relativist. True says the ancient realist. Fast forward; along comes Newton, he says “the apple is pulled to the earth by gravity.” True says the relativist. True that, and those ancient people were wrong, says the Newtonian realist. Along comes a guy with a funny haircut, he says “hey now. That Newton fellow was a smart cookie but when we do this complicated tests it breaks down. Fortunately, I discovered the general theory of relativity which solves all problems.” True — space time is curved, says the relativist. Thanks to our superior technology and experiments we can finally see reality, says the ever optimistic realist.
Point is, all these truth statements have been and continue to be useful every day and the relativist acknowledges that. What the relativist refuses to do, however, is saying that he describes reality. The relativist is too relaxed for that and he's betting that bright scientist will devise new experiments which will require new theories to explain their results. The relativist says that statements claiming to be in 1:1 correspondence to reality are a joke surely to be proved wrong by new experiments.
At this point you might think that I'm digressing. Surely the point is that thanks to science we can approximate the essence of reality. We are even converging toward it! Or so you might think. I'll look into what it means to approximate reality first, and then I'll talk about converging toward reality. To make my job easier I'll continue this only with respect to truth and reality in physics.
When one speaks of approximating the essence of reality, they usually mean one of two things. The first meaning can be that one believes to have rules and laws about the physical world which will either hold up eternally or will otherwise only be refined, never proved wrong, in the future. I'll bring this idea up again when I talk about convergence toward truth. The second understanding is that one has an as accurate model of the physical world as possible at a given time.
That one has when “using the scientific model” an as accurate model of the physical world as possible at a given time is, I believe, true. It is trivially true because scientists build their theories to explain as much observed phenomena as possible. So it's true by definition. The definition in question being one which decides which theory explains as much as possible of observed phenomena. In other words, it's a definition or theory of justification.
And this term, justification, is key because what this particular meaning for the approximation of reality is, in effect, is not truth but justified belief. Thus, what is required is a definition, a theory, of justified belief. We need such a theory without depending on 1:1 truth correspondence; we need it to not be based on the dogma of the scientific method; and we want it to be one that only tells you which belief out of many is that one belief which works best of all. This definition or theory is also, as far as I'm concerned, what skepticism is or should be about.
Mind you, as Kuhn showed (link in previous post) this would have at one point in time meant that you would have chosen an epicyclic model of the solar system over the model outlined by Copernicus. But when we don't fool ourselves into thinking we know what truth is, that shouldn't be a problem.
Next: Does our scientific body of knowledge converge to an essence of reality or not? The answer seems to be an obvious YES at first so I'll start with what science can bring us.
Science will over time, as a consequence of what I talked about concerning the first meaning for the approximation of truth, bring us a larger and larger body of knowledge that describes the observed worldly phenomena in greater and greater detail. There; bland and uncontroversially I believe that is what science will bring us. The question left is, why isn't that also bringing us closer to truth?
Well, it simply isn't obvious that there is an essence of reality. At least, it isn't known if we won't spend eternity coming up with new experiments that bring us unexpected results each time. Either because of the relation of our theories to reality, or because of a, perhaps, fundamental nature of the Universe. I won't go into the second reason because that has nothing to do with relativism.
At last it's time to give a reason why our theories may not get us to a point where we have explained everything there is to explain. Well, quite simply there are an infinite number of ways to explain any event and an infinite number of ways to explain the relations of things in an observation. Each of these ways having their own nuances. Sometimes flaws are found in a currently used theory and then a new explanation needs to be found.
This can express itself either in the growing and refining of laws in a certain school of thought, or this can express itself -violently- as the overthrowing of a paradigm and replacing it with something radically different. Like when Einstein's theory replaced Newton's explanation of gravity. This, again, is a dull fact. You'll already find many (if not most) physicists saying that Newton is a special case of relativity.
You notice this, by the way, well in psychology where there are many schools of thoughts, each with their own experimental successes and failures. In physics at the moment (though this has not always been the case; see Kuhn) there are two models accepted, being a cosmological model based on relativity and a quantum-level model. Both models are inconsistent but are also complements (how relative!). Ultimately they might both be “obsoleted” by something else; perhaps a string theory, perhaps M-theory, I don't know; which in turn would invite new experiments and so on. Anyway... whether or not there will ever come a time where we'll hit the fabric of reality, I don't dare say but we sure don't have any reason to assume we will a priori.
Another point where the one truth can be challenged are the construction of a theory that explains, for example, biology from physics. There's a lot of concepts that need to be mapped for that to happen; and the relativist's view is that many of those concepts are going to be peculiar to humans and the culture that we live and die in, i.e. not realistically translatable, or even definable in logic. And if biology works, then there's still psychology, economics, etc. The unification of all sciences, in conclusion, is very unlikely to ever happen. I wouldn't think that worldview is controversial though.
There are more arguments, of course, and I have lots more to say but I quit here with a sigh of desperation. Oh well; at least now I have experienced first-hand why people fill whole books on this topic.
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