Hume’s Enquiry - Section VII
With mathematics everything is clear and concise, but in metaphysics there is ambiguity. In order to advance metaphysics we should clear up some of its ambiguities. One ambiguous idea at the heart of metaphysics is force, or necessary connection (causes necessitating effects in the world).
All complex ideas are combinations of simpler ideas. And all these simple ideas can be traced to simple impressions. The only way we have ideas is by impressions: either impressions we get from the senses or internal impressions from the mind. These impressions are clear and unambiguous, so tracing our idea of necessary connection to its simple original impression will get rid of ambiguities.
When we observe things in nature, we see what we think are causes and effects. For example, we might see one ball collide with another ball which is at rest. Then we might see the second ball move from rest. One might reason that the moving ball and the collision caused the stationary ball to move. However, we do not observe any necessary effect from a specific cause in this situation. All we have is the sensation of seeing a ball move after seeing another ball collide with it. We do not have an impression of the second ball necessarily moving, we only observe that the second ball moves. Everything we observe in nature is like this. We see events, that seem to be triggered by other events, but we cannot say that one event causes another because we don’t have impressions of events necessarily causing other events; all we have are impressions of events taking place. We cannot discover any necessary connection by impressions of sensation.
Because we only get ideas from impressions, if we can’t find an original impression of necessary connection from sense, then it must be an internal impression. But in observing what seem to be the cause and effect relationships between objects in the world, the mind does not receive any internal impressions, only the sensory perceptions of the events. The very first time we encounter objects in the world we are not able to predict what they will do. But if we did have internal impressions of causes then we could predict what would happen in the world by rationality without ever experiencing it. Therefore, because we do not receive impressions for the idea of necessary connection internally or from sensation, we cannot have the idea of necessary connection.
This doesn’t mean that there is not necessary connection, or that there is no force that necessitates events occurring in the universe. What Hume is saying is that there might be some force that necessarily causes things to happen the way they do, but we cannot know this force, because the only way we can know something is through impressions, and we have no impressions of causes bringing about necessary effects.