For a moment, let's look at Descartes. Descartes' own response to whether or not we can know we're not dreaming was not exactly to confirm the idea that we don't know we're not dreaming. In fact, and if memory serves, Descartes thinks we can know we're not dreaming because there's a God and God is no deceiver. For a theist, this might be justification enough and so the argument could just end there, right? Well, it begs two questions: (1) If someone doesn't believe in God, it would be difficult to convince that person that this is ample justification, wouldn't it? This leads to the next question: (2) Are there other justifications for believing that we don't know we're not dreaming, or some such, than that we know there's a God and God's no deceiver?
And anyway, George Berkeley came along and argued that it could be true that God not be a deceiver and yet we not actually be seeing an external world. This is because the ultimate world could just be the infinity that is God and all this other stuff of the phenomenal world is the result of the sense experiences that God has endowed us with. They are just ways of seeing, and not actually what we see; they are mere representations in the mind.
As an aside here, I'd like to say I'm not endorsing either Descartes' or Berkeley's view, just laying out Descartes' response to his own argument and Berkeley's alternative explanation of Descartes' reason for thinking that his own argument is false.
In any event, if Berkeley's explanation could be a likely explanation, or something near enough, then Moore's argument might miss the point. Moore held up his two hands and said they were proof of two things in the world. He also argued that at least in the ordinary sense he knows that he is doing what he is doing now in the world and so knows that he's not dreaming. But, following Berkeley, a certain kind of way of looking at or responding to Moore's argument is that while it's true that Moore is using the ordinary means to arrive at evidence for there being at least two things in the world or his trusting his senses, it could alternatively be the case that he is merely having the sense-experiences of such. Therefore, with or without God's role here, it could be that the ordinary basis for evidence or for what we call knowledge is merely just representations in the mind.
Are you with me so far?
Then a fellow named Rudolf Carnap came along and argued that in some sense or other any kind of claims about the external world might ultimately be meaningless. He reasoned such by looking at sentences like this: "There are physical objects." He asked how anybody could verify such a thing in the world? If somebody already assumed that everything that there was was a physical object, it would be true by definition. But then this is just a convention that somebody has adopted to identify the things that one sees as physical objects. Carnap thought it was perfectly fine to do this, but otherwise the idea made no sense. This is because a person can't check to see if that sentence conforms to reality.
I will not examine Carnap's idea there rigorously but just acknowledge that some kind of theory of verification that Carnap had is kind of an old dogma philosophers have felt like they've dispensed with. This is not to say that his idea is wrong, only that people do not believe that this is a plausible account of how to deal with the skeptical claims.
Something along the lines of Carnap's idea persists among philosophers, however, and it was at one time advocated by philosopher and logician Bertrand Russell and still advocated today. This claim is, namely, that assuming that there are physical objects or an external world etc. is the best explanation for what we perceive and seems to do better than thinking that all we directly apprehend are representations. So philosophers try to lend support to the idea that it's just a matter of inference to the best explanation.
Philosopher Barry Stroud explains that that line of reasoning amounts to making use of a transcendental argument. A transcendental argument is when you say that for something to be, there is something else necessarily implied. For example, like for me to be able to have some sense of what it is like to be a human being, to experience pain and pleasure and so on, I must necessarily assume that other people do as well. Or to assume that I have a mind I have to assume other people have minds. These are all just examples of transcendental arguments.
Stroud and other philosophers are not convinced by these sort of arguments, for one reason because all that these transcendental arguments really establish is that perhaps we must believe that one thing necessarily entails another, like maybe we must believe that there is an external world because we can have experiences that suggest as much. This does not, according to Stroud and others, establish that they actually exist, however--again, only that we believe they do, or maybe that we must believe they do.
PF Strawson in his essay, "Skepticism, Naturalism, and Transcendental Arguments" thinks this whole debate is misguided. He appeals to philosophers David Hume and Ludwig Wittgenstein to find a coherent picture about what is going on here. Such proofs for or against an external world are idle speculation, according to Hume, and God or nature has endowed us with the idea that an external world exists. Given that we can't know beyond what we are hard-wired to believe, we should not ask questions like whether or not an external world exists but why we so believe that it does. And doing this ultimately will provide us with a more reasonable, perhaps more scientific explanation for our ways of thinking.
Hume thought there were philosophical speculations that lead us to skepticism about abstract issues (like the existence of the external world) and on the other hand commonsense speculations or intuitions which in daily life override any inclination we have toward skepticism in the abstract. Being so designed by God or nature, we cannot help but think the way we do and go on about our lives.
Wittgenstein had a different conception about the things we believe and know, thinking that what might count as an empirical fact and what might count as the scaffolding of beliefs that allow that way of thinking to flourish is not a sharp distinction. So, whether because of God, Nature or social convention (Wittgenstein actually seems to assume the latter) certain assumptions about the world existing is just part of the conventional practice and allows our thinking and interactions with what we call the world to better cohere. But it's not like we choose to affirm or deny these things. We can't help but take them for granted.
Strawson writes, in support of Hume that several beliefs we have are
outside our critical and rational competence in that they define, or help to define, the area in which that competence is exercised. To attempt to confront the skeptical doubt with arguments in support of these beliefs, with rational justifications, is simply to show a total misunderstanding of the role they actually play in our belief-systems. The correct way with the professional skeptical doubt is not to attempt to rebut it with argument, but to point out that it is idle, unreal, a pretense; and then the rebutting arguments will appear as equally idle.So this is the position that Strawson takes, ultimately: that this whole debate about whether or not the external world exists is terribly terribly misguided and we all have to accept as part of our framework. He even thinks this thinking of his is actually what philosophy ought to be doing as a discipline.
[T]o establish the connections between the major structural features or elements of our conceptual scheme--to exhibit it, not as a rigidly deductive system, but as a coherent whole whose parts are mutually supportive and mutually dependent, interlocking in an intelligble way--to do this may well seem...the proper, or at least the major, task of analytical philosophy.Lucky for him, this is what he is doing.
What do you think? Are you convinced with his position?
For my money, I am torn. But more on being torn later.