When we examine our everyday idea of causation, Hume says we find four component ideas:
- the idea of a constant conjunction of cause and effect (whenever the cause occurs, the effect follows).
- the idea of the temporal priority of the cause (the cause happens first, then the effect).
- the idea of causes and effects being contiguous (next to each other) in space and time.
- the idea of a necessary connection between the cause and the effect.
So, for instance, the idea that striking a match causes it to light is made up of the idea that whenever similar matches are struck (under the right conditions), they light, plus the idea of the striking happening first, and the idea of the striking and the lighting happen right next to each other in time and space, and, finally, the idea that the striking somehow necessitates or makes the match light. Now let’s consider these component ideas and ask whether they all have an empirical basis in corresponding sense impressions. We do have sense impressions of the first three: the constant conjunction of cause and effect, the temporal priority of the cause, and the contiguity of cause and effect. But Hume argues that we lack any corresponding empirical impression of necessary connections between causes and effects. We don’t observe anything like the cause making the effect occur. As Hume puts the point,
When we look about us towards external objects, and consider the operation of causes, we are never able, in a single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find, that the one does actually, in fact, follow the other. (An Enquiry Concerning Human Understanding, Section VII)
The idea of causes necessitating their effects, according to Hume’s analysis, is a confused projection of the imagination for which we find no basis in experience. For this reason, Hume denies that we have rational grounds for thinking that causes do necessitate their effects.
The External World
All of our reasoning about the external world is based on the idea of causation. So the skepticism that follows from Hume’s skepticism about causation is quite far reaching. Our beliefs about the external world, for instance, are based on the idea that things going on in the external world cause our sense impressions. We have no rational grounds for thinking so, says Hume.
More generally, our evidence for what we can know begins with our impressions, the mental representations of sense experience. We assume that our impressions are a reliable guide to the way things are, but this is an assumption we can’t rationally justify. We have no experience beyond our impressions that could rationally certify that our impressions correspond in any way to an external reality. Our assumption that our impressions do correspond to an external reality is a rationally unsupportable product of our imagination.
Closely related to Hume’s skepticism about causation is Hume’s skepticism about inductive reasoning. Inductive argument, in its standard form, draws a conclusion about what is generally the case, or what will prove to be the case in some as yet unobserved instance, from some limited number of specific observations. The following is an example of a typical inductive argument:
- Every observed sample of water heated to well over 100 C has boiled.
- Therefore, whenever water is heated to well over 100 C, it boils.
Unless every instance of water heated to over 100C in the history of the universe is among the observed instances, we can’t be sure that the conclusion is true given the truth of the premises. It follows that strong inductive arguments like the one above are not deductively valid. But then what justifies the inference from the premise to the conclusion of an inductive argument?