There does not seem to me to be anything more to it being "really" four feet long

I don't know what you mean, but if the board is 4 feet long, it's really 4 feet long. This is not a matter of a appearance, it is a matter of reality.

And when you say it is really four feet long, what, exactly, do you mean? If there was some dispute about its length, would you not simply take out a measuring tape (or other measuring device) and lay it next to the board, and compare the length of the board with the markings on the tape (or other measuring device)? Is there anything more to be done? Is it anything more than that?

The point is that reality and appearance are two different things.

Let's not confuse things. We are not talking about size, we are talking about the appearance of size. Things are a certain size whether we perceive them to be so or not.

What's the difference?

What is, and what we perceive to be, are two different things. Optical illusions demonstrate this. Perhaps you should have a look at those.

I dont think that optical illusions do demonstrate that. I perceive my computer and I'm pretty sure that I'm perceiving reality, if I'm not, how do you explain the appearance, to you, of this post?

we should then conclude that what we perceive, and what is real, are not always the same. That is what I mean when I say they are two different matters.

Well, perception and that which is perceived are different kinds of things, and we can be mistaken about that which we're perceiving, but generally speaking, I dont see any reason to think that we are mistaken, in most cases.

The real question here is what defines size? In terms of measurement, yes it seems that no matter where an object is in your visual field it will always measure the same length on a ruler. But in terms of how we view size, both the size of the ruler and the object - in fact even each cm on the ruler - are going to change as we view them, so even saying something is 10cm means nothing, except as in relation to other objects in the world.

Well, perception and that which is perceived are different kinds of things, and we can be mistaken about that which we're perceiving, but generally speaking, I dont see any reason to think that we are mistaken, in most cases.

In order for us to perceive the object as 10cm, we need relation (if I'm not mistaken - I was just reading an article on perception). But the object would still be 10cm, if it was in fact 10cm, no matter if we perceive it to be or not. There is an objective world which exists no matter how it is perceived.

illusions show us that often what we perceive is not true to reality, which leads to scepticism about the validity of our perceptions.

i mean, does saying something is 10cm give us any useful information about its true size?

You should read the Meditations, i think your view will be slightly different after that...
But to summarise Descartes, illusions show us that often what we perceive is not true to reality, which leads to scepticism about the validity of our perceptions. He also cites the example of hallucinations and dreams, often when we are having a vivid hallucination or dream, we are unaware that what we are seeing is not reality, so how can we be sure that we are not dreaming or hallucinating right now. Finally, he cites the example of an evil deceiver. This is a matrix like scenario, where an all powerful deceiver is controlling all our perceptions, and projecting them into our minds like a film. What we are seeing is qualitatively indistinguishable from reality, so how are we to know that it is not reality?
This is the problem we are faced with about the reliability of our perceptions, do they really show us what is there, in an independent world, or is everything we are seeing just inside our minds?

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It doesn't lead me to such skepticism, because some As being Bs doesn't imply that all As are Bs. And I would reject your "often".

The argument:

1. We are sometimes mistaken

Therefore (2) we may always be mistaken.

Is invalid.

And even Descartes recognizes that, because after he makes that argument, he points out that the conclusion does not follow, and he goes on to the dreaming argument.

Yes, it does. We usually perceive reality accurately. If measurements didn't give us any useful information on true size, how do you suppose things like skyscrapers are built? Do you have any idea how precise some of those measurements must be?

But the fact remains that we cannot reach that external world because of the veil of perception

Yes, the illusion argument is fairly weak due to the irregularity of illusions, and i'm not 100% advocating it, just pointing it out. However the final total deception argument is stronger. There really is no way of telling weather or not what we are percieving right now correlates with any mind independent reality.

But what makes you refer to it as a "veil"? Why not refer to it as a "window", as we are often perceive reality accurately?

But what makes you refer to it as a "veil"? Why not refer to it as a "window", as we are often perceive reality accurately?

Oh, I don't think it is that dire. We cannot do it with complete certainty, but that does not mean we cannot do it with something less than certainty. We can have varying amounts and kinds of evidence. I am fairly sure that there are pigeons in Trafalger Square, and that I am justified in being sure there are.

But the fact is we don't know we're perceiving anything accurately, again, check out Decartes' total deception argument, or just watch the Matrix! In either scenario what we are seeing is not reality, but it looks just like reality, so we simply cannot tell the difference.

Kant also says we can't even talk about reality, because we have our own conceptual scheme which means we project certain qualities onto the world that aren't really there. For example, the fact that things do get smaller as we move away from them, this doesn't happen in reality, but if our minds didn't do this, the world would be unperceivable. He uses the example of rose tinted sunglasses that we can't take off, our minds are like the sunglasses, they make the world seem different to how it really is.
I'm not explaining very well... try this In As philosophy; how does Kant's conceptual scheme work? - Yahoo! UK & Ireland Answers