What motivated Berkeley to develop his idealist system?

Berkeley explicitly cites his hatred of skepticism and atheism and the need to combat those two forces as his overriding motivation in developing an idealist system.

On what grounds does Berkeley oppose atheism?

Berkeley's hatred for atheism is self-explanatory; as a religious believer, Berkeley did not like his era's growing disbelief in God.

What were the skeptics that Berkeley opposed skeptical about?

Skeptics had doubts on several key questions, including whether their perceptions of reality matched the way reality actually is, and knowing the true nature of things, as opposed to simply knowing their subjective impressions of these things. Skeptics had also come to believe that our senses ultimately deceive us about the true nature of reality.

Why does skepticism bother Berkeley so much?

It is highly likely that Berkeley’s hatred of skepticism was caught up in his fear of encroaching atheism. However, in and of itself, he felt that the doubts implicit in skepticism were completely contrary to common sense. He believed that no sane person could really doubt that there is a world external to their own perceptions, and that roughly corresponds to those perceptions. In Berkeley’s view, few people would seriously entertain a notion as absurd as, say, the possibility that an evil demon is causing all of our sensations of the world (a doubt which Descartes raises and then discards). Similarly, he thinks, it takes much philosophical training to seriously entertain the equally absurd belief that real objects are not colored, odorous, full of sound, and taste, and so on. Any person with common sense knows that the real world is just as we perceive it to be, and any person with common sense trusts his or her senses.

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