Hume, lecture #1 (pp. 1-13)

1. pp. 1-9 is basically a speech to the reader, in which Hume says that he’s going to try to figure out how the mind works, emulating Newton’s (then) recent establishment of the laws of mechanics.

2. All ideas are less `forceful and vivacious’ copies of sensations. Hume here intends a quite literally mechanical model of thought.

3. Hume will use the term `impressions’ to apply to all perceptions (including perceptions of inner states). All mental states are divided by him into these two main classes: ideas and impressions.

4. Hume claims that all ideas are just copies of impressions. Some ideas compose different impressions together. For example, we can form the idea of a unicorn because we’ve had impressions of both horns and horses.

5. Hume provides two arguments for this claim. The first has just one premise: that no one can find an example of an idea for which there is no impression from which it can be derived. Note that he’s here denying the central claim made by both Plato and Descartes.

6. The second argument is based on the premise that where particular individuals lack access to certain types of impressions, they also lack the corresponding types of ideas. As a general claim we now know this to be false, though it’s perhaps true with respect to some ideas. We need to carefully distinguish two claims here:

(i) If a person lacks a certain type of impression, he may, as a result, not have the corresponding idea.

(ii) If a person lacks a certain type of impression, then he cannot form the corresponding idea.

Claim (i) is obviously true. But it’s (ii) that Hume is defending.

7. Hume admits one exception to the principle. Suppose, he says, that there’s one particular shade of blue you’ve never happened to encounter. If presented with the two shades on either side of it along the color spectrum, could you not infer it? Yes, Hume thinks. So this, he suggests, is a singular exception to his general principle.

8. How can Hume allow an exception like this, and then just blithely move on? The answer is important: Hume is not, like Descartes, seeking logically certain principles. It’s quite good enough for his purposes if his principle is just a generalization, along the lines of `Dogs have four legs.’ We don’t withdraw the generalization just because we know there’s the odd three-legged dog.

9. The final paragraph in the main text on p. 13 is, basically, a shot at Descartes.