[T]he unanswerable question whether, or to what extent, any picture our senses "convey" might correspond to the "objective" reality is still today the crux of all theory of knowledge. Sextus used, among other things, an apple as an example. To our senses it appears smooth, scented, sweet, and yellow -- but it is far from self-evident that the real apple possesses these properties, just as it is not at all obvious that it does not possess other properties as well, properties that are simply not perceived by our senses. The question is unanswerable, because no matter what we do, we can check our perceptions only by means of other perceptions, but never with the apple as it might be before we perceive it.
----- E. von Glasersfeld, “An Introduction to Radical Constructivism”
Here's my best reconstruction of Ernst's rejection of realism:
1. All concepts are in the mind
2. All concepts are conceptualized
3. All concepts are concepts-of-some-x
4. All concepts-of-some-x are in the mind (from 1)
5. There can be no unconceptualized concepts-of-some-x (from 2)
6. All concepts of apples are concepts-of-some-x (from 3)
7. There can be no unconceptualized concepts of apples (from 5 and 6)
8. There can be no concept of an apple that is not a concept (truism)
9. The referent of the realist’s phrase “unconceptualized apple” is not (simply) a concept (an essential assumption of realism)
10. There can be no concepts of unconceptualized apples (from 8 and 9) (von G.: “…we cannot possibly conceive of an unexperienced world”)
11. Metaphysical realism (MR) involves concepts of unconceptualized apples. (By definition; here’s a typical realist concept: “if every concept-user were suddenly to disappear, apples – unconceptualized and unconceptualizable – would still exist.”)
12. MR is wrong and von Glasersfeld’s constructivism right (from 10 and 11)
"sheep in the field" case is similar to the example I raised in
The sheep in the field (Chisholm 1966/1977/1989). Imagine that you are
standing outside a field. You see, within it, what looks exactly like a sheep.
What belief instantly occurs to you? Among the many that could have done so, it
happens to be the belief that there is a sheep in the field. And in fact you
are right, because there is a sheep behind the hill in the middle of the field.
You cannot see that sheep, though, and you have no direct evidence of its
existence. Moreover, what you are seeing is a dog, disguised as a sheep. Hence,
you have a well justified true belief that there is a sheep in the field. But
is that belief knowledge?
And, finally, a snippet from Berkeley-the-idealist (von
The table I write on, I say, exists, that is, I see and feel it; and if I were
out of my study I should say it existed, meaning thereby that if I was in my
study I might perceive it, or that some other spirit actually does perceive it.