Humans also have the ability to look at two ideas simultaneously without combining the; Locke calls these ideas of relations. Finally, abstraction occurs when ideas are separated form all other ideas that generally accompany them in experience. In this manner, Locke believes he has completely described and defined all types and aspects of human thought. Berkeley, like Locke, believes that it is only through sensation or experience that we can attain any knowledge about the world around us. He goes somewhat further, however, in describing the way these sensations work, claiming that most ideas that Locke would have called "simple" are really complex lists of simple ideas combined into one larger idea -- the idea of an apple, for instance, is a combination of many different ideas regarding shape, color, taste, smell, etc. (Stanford). Berkeley also reverses, in a way, the argument made by Locke. Whereas Locke believes that the mind's ideas are shaped by -- indeed, are created by -- experience of things by the senses, Berkeley claims that the existence of these things is dependent on an observational mind: "That neither our thoughts, nor passions, nor ideas formed by the imagination, exist without the mind, is what everybody will allow. And it seems no less evident that the various sensations or ideas imprinted on the sense, however blended or combined together...cannot exist otherwise than in a mind perceiving them" (Berkeley, 30).

Berkeley's argument is more complex than Locke's; he claims that it is absolutely pointless to assume reality without observation. Ideas, then, are not only shaped by experience, but even the means by which experience exists. David Hume does not entirely agree with this rather radical claim of Berkeley's. Hume notes that ideas in memory and imagination differ greatly from ideas tat occur during direct sensation. In addition, Hume claims that those who cannot personally experience a given sensation, rather through...

This would seem to disagree with Berkeley's claim about reality being shaped by ideas; though mild-mannered men by not be able to comprehend rage, they would certainly not disagree that it exists in others. Thus, Hume determines that ideas are shaped by experience, but that not all experience is fully capable of shaping ideas. He also points out a flaw in the empirical argument, where a man might be able to imagine a shade of a certain color that he hasn't seen, but this could be explained with Locke's concept of complex ideas and their formation from simple ones.
The argument put forth by these three men are similar in very important ways; all agree that there can be no knowledge without experience. But the nature of reality itself is thought of differently by each. Locke believes that the world is as we see it, while Berkeley believes the world exists because we see it. Hume is less concerned with the metaphysical aspects of the nature of thought, but his notion of how ideas are formed is in some ways more complete, as he includes the contradictions of his argument.