Empiricism (en- peiran; to try something for yourself): The doctrine that all knowledge must come through the senses; there are no innate ideas born within us that only require to be remembered (ie, Plato). All knowledge is reducible to sensation, that is, our concepts are only sense images. In short, there is no knowledge other than that obtained by sense observation.
Remember that according to Descartes, what I know first and foremost are my ideas. It is only later that he seeks to know if the extramental world exists, and so he begins with his ideas and then moves towards real being (rather than vice versa). Somewhere along the line the notion of idea undergoes a transformation. Soon an idea becomes a sense impression or an image. Remember that for the Greeks and Mediaeval thinkers, an idea is not reducible to an image. An idea cannot be imagined, but is an essence abstracted from the phantasm and understood. But this gets confused after Descartes. For example, John Locke says that ideas "...stand for whatsoever is the object of the understanding when a man thinks, I have used it to express whatever is meant by phantasm, notion, species, or whatever it is which the mind can be employed about in thinking..."
Note how he lumps together "phantasm, species, or whatever". This is very sloppy, but influential nonetheless. And notice how he maintains that the object of our knowledge is the idea, and not real being (as it was for the Greek and Mediaeval thinkers).
David Hume, following this line of thinking, begins by distinguishing the contents of human experience (which is ultimately reducible to perceptions) into: a) impressions and b) ideas.
Impressions are given sensations that arise from "unknown causes". Remember that what we know are our impressions, according to this trend. Whether there is something that corresponds to these impressions is unknown, for we don't know real being, we know impressions (a la Descartes).
Ideas are man's thoughts. They are fainter copies of impressions, and so they are images in the imagination that are remembered.
Every idea stems ultimately from a corresponding impression. Complex ideas are ultimately reducible to impressions; for they can be analyzed into simple ideas, which in turn can be reduced to impressions.
The consequences of these principles are important to note. That is why Hume is important, for he shows us where Empiricism ultimately leads.
1) The Rejection of the Principle of Causality: The idea of cause and effect is groundless, according to Hume. In the idea of causality, it is maintained that there is a necessary connection between the effect and its cause. The baseball moved across the plate because of (the cause being) the rapid movement of my arm. The eight ball moved because (the cause being) it was struck by the white ball, which was moving. We say that there is a relationship between what we refer to as the cause and what we refer to as the effect, and that relationship is not merely contingent (may or may not follow the movement of my arm or the movement of the white ball), but is rather necessary. The white ball will necessarily move, unless there is something causing it to do otherwise (ie, it is glued to the pool table). But if ideas are nothing but faint and remembered impressions (perceptions), what is the impression or perception that is at the root of such an idea as causality? We see one impression, namely the movement of the white ball, and this is followed by another impression, the movement of the eight ball. But where is the perception of the necessary connection between the movement of the white ball and the movement of the eight ball? There is none. So why then do we talk of cause and effect? How do we know that the white ball caused the eight ball to move? Remember, all knowledge is reducible to sense perceptions. "There is a transfer of energy between the white ball and the eight ball", says the physicist. Hume replies: "There are no ideas, which occur in metaphysics more obscure and uncertain, than those of power, force, energy or necessary connection, of which it is