By making real things into ideas, Berkeley thinks that he has posited a world system that is immune to skepticism. Skepticism infiltrates a system in two ways: it raises doubts about whether anything really exists, and it raises doubts about whether appearances adhere to reality. Neither of these doubts can be raised on Berkeley's picture. Since real things are just sensations, once you have a sensation of, say, a tree, it is a contradiction to doubt that the tree exists. The tree is just the sensation, and you cannot doubt that you had that.

In addition, there is no room for wondering whether the tree is really like it appears to you. There is nothing to the tree other than your sensation of it. There can be, therefore, no distinction between appearance and reality.

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