Friday, January 30, 2015

Hume Policing the Frontier Between the Observed and the Unobserved

Our judgments about cause and effect, according to David Hume in his Treatise of Human Nature, are nothing more than projections of our sense impressions, prompted by the "force and liveliness" of those impressions.

But where do the debutantes of forceful and lively impressions go after their first presentation to the monarch of our mind? They subside on the couches of the Memory and the Imagination, says Hume, and memory is "much more lively and strong than those of the imagination." A "perfect idea" is an impression that has entirely lost its vivacity. Thus banished to a nether world without the strong light of vivacity, our perfect ideas in the imagination are subject to the fancy and may stray to thoughts of "winged horses, fiery dragons, and monstrous giants." The natural thing is for ideas of the memory to fade away and become mere imagination. But sometimes ideas of the imagination can reacquire force and vivacity and counterfeit as memory. It is the practice of "liars", says Hume, by frequent repetition of their imaginary ideas, to end up believing them as reality.

Obviously, we cannot use the perfect idea of the imagination in any process of inferring from the observed to the unobserved.

But Hume famously goes further than this. He argues that there is no basis in reason to infer the unobserved from the observed at all. All we can argue is "constant conjunction." When we experience impressions in constant conjunction we come to believe they are related, from the force and vivacity of the impressions and their constant conjunction.

But notice that Hume does have a theory of cause and effect. He assumes that when people see a constant conjunction their minds are driven to assume that if one event occurs the other must necessarily follow. So he does believe in reason. The other approach is to assume that everything is contingent, that mental conjunctions are just as unprovable as conjunctions between forceful and vivacious impressions, that everything is merely a tissue of unprovable assertions and predictions.

The modern approach is the way of settled science. The more that theory about separate impressions gets confirmed and the more that predictions about cause and effect are successful, the more we are justified in putting our faith in their causal connection. But you never know. The practical thing to do is assume that everything works as advertised. Until it doesn't.

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