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Hume says the idea of a cause seems to be the idea of two things being connected, such that if one happens, so does the other.
He refers to this as a 'necessary connection'.
But if this is what we mean to assert when we say A causes B, says Hume, we can never have any proper grounds for making such an assertion.
Hume's objection is that we can never have any perceptual grounds for a claim that two things are necessarily connected.
All we can observe as we study a pair of events is that at most they always go together.
Hume says it must therefore be on the basis of observing such constant conjunctions that we assert causal relationships.
The constant conjunctions do not warrant our assertion of necessary connection.
It is Hume's thesis that we see the constant conjunction and move to the conclusion that there must be a necessary connection. This movement from the observation to the conclusion that there is a necessary connection is a matter of psychology, in Hume's view. There is no warrant for the move. It is a logically unjustified leap.
But what exactly is it to say two events are 'necessarily connected'?
Hume answers this by interpreting the question in a Lockean way. To understand what a word means we need to think of the idea it stands for.
So we can ask the Lockean question: What is the origin of our idea of necessary connection?
It comes, he says, from a habit.
I constantly seeing B immediately after seeing A.
This regular experience results in the following: my mind, when it sees A moves of itself to the thought of B.
It doesn't wait for the sight of B.
It moves on its own to the thought of B.
If I introspect on this activity of the mind, I see an association between two ideas, the idea of A and the idea of B.
It is this which gives me the idea of necessary connection.
Our minds move from one idea to another. We introspect on this. Then we think: the world is like that. One thing leads onto another.
Our belief in causality is a projection onto the world of a habit of our minds.
'Suppose a person, though endowed with the strongest faculties of reason and reflection, to be brought on a sudden into this world; he would, indeed, immediately observe a continual succession of objects, and one event following another; but he would not be able to discover anything farther. He would not, at first, by any reasoning, be able to reach the idea of cause and effect ... nor is it reasonable to conclude, merely because one event, in one instance, precedes another, that therefore the one is the cause, the other the effect. Their conjunction may be arbitrary and casual. There may be no reason to infer the existence of one from the appearance of the other. And in a word, such a person, without more experience, could never employ his conjecture or reasoning concerning any matter of fact, or be assured of anything beyond what was immediately present to his memory and senses.
Suppose, again, that he has acquired more experience, and has lived so long in the world as to have observed familiar objects or events to be constantly conjoined together; what is the consequence of this experience? He immediately infers the existence of one object from the appearance of the other. Yet he has not, by all his experience, acquired any idea or knowledge of the secret power by which the one object produces the other; nor is it by any process of reasoning, he is engaged to draw this inference. But still he finds himself determined to draw it: and though he should be convinced that his understanding has no part in the operation, he would nevertheless continue in the same course of thinking. There is some other principle which determines him to form such a conclusion. This principle is Custom or Habit.'
Hume, Enquiry, V, I
