All reasoning concerning matters of fact seems to be founded on the relation of cause and effect. By means of that relation alone we go beyond the evidence of our memory and senses.

Concerning the nature of that evidence which assures us of matter of fact, we must enquire how we arrive at the knowledge of cause and effect. The knowledge of this relation is not, in any instance, attained by reasonings a priori.

Such is the influence of custom that, where it is strongest, it not only covers our natural ignorance, but even conceals itself, and seems not to take place, merely because it is found in the highest degree.