In addition to the arguments outlined above, Berkeley also has another argument which he thinks proves conclusively that materialism is false. This argument seeks to demonstrate that it is inconceivable for an object to exist outside of the mind. The argument goes like this:

(1) We can conceive of a tree existing independent of an out of all minds whatsoever only if we can conceive of the tree existing unconceived.

(2) But an unconceived conceived thing is a contradiction.

(3) Hence, we cannot conceive of a tree (or anything else) existing independent and out of all minds.

In plainer terms: To conceive it possible for a tree to exist outside of all minds, we need to be able to think of an unconceived tree. But as soon as we try to conceive of this unconceived tree, we have conceived it. So, we have failed. Try to imagine, for instance, a tree deep in some primeval forest. Surely this tree has never been conceived. But it just was. As soon as you imagined it, it was conceived.

It is agreed by almost everyone that this argument is terrible. But people disagree about exactly what mistake Berkeley is making here. Most people think that Berkeley's problem is that he fails to distinguish between the act of perception and the content of the perception. It is one thing for me to be imagining the tree, and quite another for the content of my imagining to be of an unconceived, unperceived tree. According to this analysis, Berkeley is right to point out that it is a contradiction to assert that there is some X that is both unconceived and is also conceived by me. However, he is overlooking the fact that the content of the conception can be isolated from the act of conception. I can have a conception, the content of which is: unconceived tree.

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