The first important concept that Berkeley established is that "to be" means the same thing as "to be perceived". This is because, he argues, all things are only materials because people are there to perceive them. Ideas are things that are perceived as well, therefore, if material things can be perceived, then they are also ideas.
Berkeley sharply disagrees with the ideas of his fellow philosopher, John Locke, when it comes to perceiving objects. While Locke says that all things are made up of matter, which is a senseless state, Berkeley says that senseless things cannot be perceived, therefore they cannot be made of matter. Because they are not made of matter, they are made of ideas.
The qualities of object are also, he claims, ideas. For example, the size, shape, and color of something are all relative to the person viewing the object, therefore the object must be made of ideas. One person may view something that one person thinks is green as blue, and will never know the "true" color of the object. Since everyone views things differently, there are no definite ideas of an object, but, nevertheless, objects are made of ideas.
