(Note: I’ve decided to start posting some lectures I’ve written out, under the new category “from the old yellowed notes”.)

We can examine our lives from the inside or from the outside. From the inside we jump from thought to thought, from sensation to sensation, with memories mixing in with new ideas, interrupted by our notice of this or that. We may call our consciousness a stream, but it is in truth a confluence of many streams with complicated turbulence, backflows, and eddies. James Joyce captured “life from the inside” in Ulysses and Finnegan’s Wake, which together show the strangeness of what is most familiar to us.

From the outside we can chronicle what we were doing last evening, what we did this morning, and what we are doing now. We construct a timeline of events and fit our experiences into it. If we really want to, we can also fit into our timeline what was happening or being experienced elsewhere by other people. In this way we construct an objective sequence of things – the world, really – and usually we take this sequence to be the real story, and we try to fit the inside stream into it.

But in some sense this gets things the wrong way around. The inside stream is what we experience – arguably, it is all we ever experience. The outside timeline gets constructed out of the inside stream. Indeed, if we are empiricists, and wish to stay as closely as possible to experience, we may wonder whether the outside timeline has any being at all apart from the inside stream; it exists only occasionally as episodes within the inside stream.

Just as Isaac Newton sought the general laws and patterns that successfully described the changes in orbiting bodies over time, a student of human nature – like David Hume – might seek to discover the laws and patterns of experiences within the inside stream. From these laws and patterns it may be possible to establish the laws and patterns of objects in the outside timeline. For, again, we are the ones who construct the outside timeline from the inside stream.

Generally, Hume believed, impressions provide the basis for all experience, and we form both memories of these impressions and ideas based upon them. The only intrinsic difference among impressions, memories, and ideas is in how vivid they are. Natural links or relations form among them, depending on resemblance, contiguity in time or space, and causation. Furthermore, as we like, we can see further relations among them, depending on quantity, quality, identity, and contrariety.

These relations for the most part guide us safely in putting structure into our experience. But the most important of them – causality – presents us with a puzzle. First let me explain that it is the most important relation because, when you think about it, the only way to turn the inside stream into an outside timeline is by positing a stable world of people and things in ongoing causal relationship with one another. I might experience first the smoke, and then the fire; but I will know that, in the outside timeline, the fire came before the smoke only if I know that the fire causes the smoke. (Kant will have much more to say about this in CPR.)

But causality is puzzling because we never really experience it. Suppose a big yellow bird falls on a silver trash can and there follows a great noise. What is the cause of the great noise? The collision of bird with can? Or the collision of yellow with silver? Of course, we can experiment. We can try collisions among other yellow and silver things; we can paint the trash can, and even paint the bird, and repeat many times. At the end, what we will find? Probably, that there is a correlation between collisions of heavy objects and great noises. But as we know, correlation is not the same as causation. There certainly is no logical link between a cause and its effect, and all experience shows is that, so far as we have observed, the “cause” and the “effect” usually accompany one another.

What this means is that, when we seek causality, we only ever find contiguity between kinds of events in space and time. Of course, we feel like we find more than this. We feel like we discover some kind of necessity connecting a cause with its effect. But Hume argues he can find no evidence of this necessity in our experience. Where then does this feeling come from? Hume suggests it can only come from us, from our own psychologies. We become so accustomed to experiencing the pair -(Cause, Effect) – that we confuse our own very strong but inward expectation with a genuine necessity existing between two events. We project our feeling of necessity onto the world. But while this explains why we might believe one event causes another, it does not give us any knowledge that it does.