Today we discussed David Hume's argument regarding necessary connections. Hume points out that our knowledge of cause and effect is knowledge that we have a posteriori, meaning that is knowledge we only get after we observe things in the world. What Hume means is that causal regularities are not logical laws; there is nothing contradictory about thinking that things function very differently from the way they seem to function in the world. This means that if we have knowledge of causality, it must come from our observations of the world.
Hume's copy principle states that every idea is a copy of an impression. So, Hume can ask, do we have impressions of causal power? Hume goes looking for causal power in a philosophically very robust sense, as a necessary connection between cause and effect. The question for Hume then is, do we (ever) observe a necessary connection between a cause and an effect? If the answer is no, then we have no impression of a necessary connection. Hume thinks that you can see that the answer is no by considering your own experience. You aren't aware of an impression of necessary connection between a cause and an effect, are you? If not, then the copy principle tells us that we can't have any ideas of necessary connection either.
Philosophers in Hume's time tried to support a necessary connection view in various other ways, and Hume attacks their accounts directly.
A mind-body dualist, like Descartes might say that the soul causes the body in a necessary way. Hume has three replies: