Hume's Enquiry

In Hume's "Enquiry Concerning Human Understanding", we are introduced to (perhaps reminded of, but in new terms) the concept that there are two very distinct mental processes: impressions and thoughts or ideas. Impressions occur through experience; for example, we have the impression of pain when we try to pick up an ember from a fire, and the impression of red occurs when we see the blistering skin afterward. Impressions occur when something in the natural world impresses upon us, regardless of how we think or feel about that something. We hardly have any control over the impression itself, but our thoughts about the impression are indeed our own. With this in mind, all of our thoughts must originate from some impression or multiple impressions, for we can not think about something that has not made an impression upon us. Even when we invent mythical and fantastical worlds and beings that seem to defy any conventional knowledge, when broken down they are all just combinations of things we have already had impressions of from the natural world. With this understanding, we can move on to our discussion of causality.

Using Hume's billiards example, how do we know that when one billiard ball strikes another with enough force, the second will move? We don't know this a priori. There's no truth separate from experience that tells us that second ball with move when struck. We know it will only because we have seen it happen before, or seen something similar. We need not have seen billiard balls move in this way, any objects will suffice. Furthermore, the movement of one billiard ball certainly does not imply the movement of any others. For example, suppose we have ball A moving across the table with ball B at rest. We can't say that ball B will begin to move because ball A is in motion because there is no requirement that ball B move when ball A does. If both balls were moving across the table, we could notice the appearance of ball A sending ball B into motion, but there's something happening in between ball A's motion and ball B's rest that, according to Hume, we can not rightly call a "cause".

Here I have to disagree with Hume. He is so spot-on about so much, but his theory of constant conjunction vs. necessary connexion seems misguided. Yes, humans are certainly conditioned by seeing things in conjunction, but there are things that are necessarily connected in nature that are not just observations of conjunction. Water molecules, for example, are a combination of two hydrogen atoms and one oxygen atom. Nature can not change the rules here. Adding or subtracting atoms will alter the molecule, making it something other than H2O. This is a necessary connexion: two parts hydrogen and one part oxygen results in water, not just an observed conjunction.