Our notion of causality, that a particular effect is necessarily produced by a particular cause, is the result of our association of ideas, a habitual or customary observation of certain phenomena which appear to have such relations. These apparent relations do not carry with them any necessity or universality. Sensations or impressions are separated from one another and so do not have in them anything universal or necessary. What is open to us is only a probability and no certainty. Particular causes may not produce particular effects.
Causality rests on mere instinct or belief.
We do not know of any uniformity, regularity or certainty in the working of Nature. Everything becomes a matter of doubt. We are limited to perceptions and images. Though we are accustomed to observe causal relations among our ideas and perceptions, we do not see any ground for supposing this relation between perceptions and objects. They are nothing.