The missing coin is consistent with idealism and materialism. It's not evidence for either of them.
Again, that can happen under idealism as well as it can under materialism.
It's not daft at all. If the evidence (and I assume you're referring to what we perceive with our senses) is consistent with both idealism and materialism, then how can it be evidence for one or the other? Suppose I tap on my desk. I hear sounds and feel a hard surface (sensory evidence (or sense-data)). But why should I suppose those sensations can only occur with a material object? I'm certain you've had dreams that seemed very real to you and you weren't aware you were dreaming. Your senses seemed to work in these dreams, right? It's equally possible that the sensations I'm getting when I tap on a desk are the result of a very vivid dream.
Man I hate to get back into this, but..........
What I think you misunderstand about Kevin's statements above and what David and I and many others have been trying to say to you is this:
the evidence of our senses is not evidence of a particular ultimate reality. None of us, as far as I know, even speaks about ultimate reality having any set characteristics that we can define. The evidence of our senses is evidence of our senses, so when we say that we have evidence of the reality of a coin, that is all that we mean -- that coin follows certain rules.
When it comes to an ultimate reality, there is either monism, dualism, or pluralism. We approach this from a monist perspective. If monism is correct, then we can't say anything about the ultimate substance because words are defined in relation to one another. The only thing we can do is discover the rules by which what we see works.
If dualism is correct, then there is an interaction problem. How does the mind of God create the world? That's easy in a trivial sense, through thought. But the problem then becomes, what is God? If He is Other, then his mind does not work by the rules that we see around us (if it did then he would just be part of the same rule following thought-stuff), so how does he create the world? What is "mind"? We are left with only one answer -- magic, which means we can't understand it. That's perfectly fine with me as an explanation, though non-parsimonious and a little unsatisfactory. It is certainly possible, and most monists would say that we can't tell the difference anyway, so it doesn't matter. A consequence, though, is that "we" are just rule following bits of the mind of God doing its thing, so we are the same as a chair, in a sense -- everything is the mind of God. If everything is the mind of God, then our thoughts when they come to God are just God thinking itself, not "us" realizing God in some cosmic sense.
If you think that "we" have some separate form of free will, then "we" are something different from God and also different from chair, which would seem to imply neither monism nor dualism but pluralism. Why do we want to keep multiplying the "stuff" of the universe?
It would seem to me that even from the perspective of idealist dualism there is only one being with free will and that is the Ultimate Mind. Everything else is some form of his thought, so an illusion. I'm not sure how this helps anyone or differs in any way from a monist perspective. Hence David's frequent reply, "what difference does it make, since it's all the same anyway?"