Reading : Western Philosophy II.6 "Nothing Outside the Mind"
Berkeley disagrees with Descartes and Locke, who treat extension as being a quality which has resemblance to our idea of it. "An idea can be like nothing but an idea; a colour or figure can be like nothing but another colour or figure. If we look but ever so little into our thoughts, we shall find it impossible for us to conceive a likeness except only between our ideas".
Berkeley says that an idea is something which is perceived by a thinking mind, and so cannot exist if no mind is there to perceive it. Ideas can be evoked in us in different ways:
This position is called idealism, the claim "that the physical world is in some way dependent on the conscious activity of humans" (Okasha). Realism, on the other hand, says that the physical world exists independently of human perception.
This position raises a couple of questions:
Berkeley says that since these ideas cannot be created by material substance, and since they are not imagined by us (we can't conjure them up, or make them disappear by force of will), then they must me created in us by God: "There is therefore some other will or spirit that produces them".
Berkeley admits that there is a coherence, or predictability, in the way that objects behave - if we leave a room with a chair in it, and re-enter the room later, the chair will still be there, even if in the meantime no-one was conscious of it. This predictability he ascribes to God "the admirable connection whereof sufficiently testifies the wisdom and benevolence of its Author".
The other way that Berkeley addresses this is to say that an object would exist if we were to go and perceive it. "When in broad daylight I open my eyes, it is not in my power to choose whether I shall see or no, or to determine what particular objects shall present themselves to my view"