Today we encountered George Berkeley's 'immaterialism'. Berkeley's point is that there is no such thing as matter. Some philosophers think that perception is a three part event, involving a perceiver, who perceives ideas, which represent bodies. Berkeley aims to simplify this story, by proposing that bodies = collections of ideas. So on Berkeley's view, a tree or a desk is a combination, not of physical atoms, but of very basic ideas.
Berkeley offered at least two reasons for taking his view.
The first reason is an argument from a Lockean point of view. Locke thought that ideas of primary qualities resemble their objects and reveal an external world, while ideas of secondary qualities do not. Locke thought that we could see this because our ideas of secondary qualities vary from one perceiver to another, while our ideas of primary qualities don't vary. But Berkeley points out that our ideas of primary qualities vary also. For example, figure looks different from close up or far away, and from different vantage points something may or may not seem to be moving. Berkeley's point is that, for all the same reason that Locke thinks that ideas of secondary qualities are mind-dependent, he (Berkeley) can conclude that ideas of primary qualities are mind independent as well. For this argument, check out sections 14-15.
The second reason is what I call 'Berkeley's challenge'. If you disagree with Berkeley that there is no material world and only ideas, Berkeley challenges you, the reader, to imagine something that is not an idea. Isn't anything you might think of an idea? Berkeley thinks you will find the answer is yes, and if that is true, then you can't really present an alternative to his view. If Berkeley has a view but you don't, Berkeley thinks he wins by default.