"In the time of Hume there was a widespread belief in angels. That is, human figures with wings. Have you ever seen such a creature, Sophie?"
"No."
"But you have seen a human figure?"
"Dumb question."
"You have also seen wings?"
"Of course, but not on a human figure."
"So, according to Hume, an 'angel' is a complex idea. It consists of two different experiences which are not in fact related, but which nevertheless are associated in man's imagination. In other words, it is a false idea which must be immediately rejected. We must tidy up all our thoughts and ideas, as well as our book collections, in the same way. For as Hume put it: If we take in our hands any volume ... let us ask, 'Does it contain any abstract reasoning concerning quantity or number?' No. 'Does it contain any experimental reasoning concerning matter of fact and existence?' No. Commit it then to the flames, for it can contain nothing but sophistry and illusion."
"That was drastic."
"But the world still exists. More fresh and sharply outlined than ever. Hume wanted to know how a child experiences the world. Didn't you say that many of the philosophers you have heard about lived in their own world, and that you were more interested in the real world?"
"Something like that."
"Hume could have said the same thing. But let us follow his train of thought more closely."
"I'm with you."
"Hume begins by establishing that man has two different types of perceptions, namely impressions and ideas. By 'impressions' he means the immediate sensation of external reality. By 'ideas' he means the recollection of such impressions."
"Could you give me an example?"
"If you burn yourself on a hot oven, you get an immediate 'impression.' Afterward you can recollect that you burned yourself. That impression insofar as it is recalled is what Hume calls an 'idea.' The difference is that an impression is stronger and livelier than your reflective memory of that impression. You could say that the sensation is the original and that the idea, or reflection, is only a pale imitation. It is the impression which is the direct cause of the idea stored in the mind."
"I follow you--so far."
"Hume emphasizes further that both an impression and an idea can be either simple or complex. You remember we talked about an apple in connection with Locke. The direct experience of an apple is an example of a complex impression."
"Sorry to interrupt, but is this terribly important?"
"Important? How can you ask? Even though philosophers may have been preoccupied with a number of pseudoproblems, you mustn't give up now over the construction of an argument. Hume would probably agree with Descartes that it is essential to construct a thought process right from the ground."
"Okay, okay."
"Hume's point is that we sometimes form complex ideas for which there is no corresponding object in the physical world. We've already talked about angels. Previously we referred to crocophants. Another example is Pegasus, a winged horse. In all these cases we have to admit that the mind has done a good job of cutting out and pasting together all on its own. Each element was once sensed, and entered the theater of the mind in the form of a real 'impression.' Nothing is ever actually invented by the mind. The mind puts things together and constructs false 'ideas.' "
"Yes, I see. That is important."
"All right, then. Hume wanted to investigate every single idea to see whether it was compounded in a way that does not correspond to reality. He asked: From which impression does this idea originate?