The probabilities of causes are of several kinds, but all come from the same source, namely the association between a present impression and certain ideas. As the habit that produces the association comes from the frequent conjunction of ·kinds of· objects, it ·can’t spring into existence all at once, but· must arrive at its full force gradually, gaining new force from each instance that we observe. The first instance has little or no force, the second adds a little to it, the third becomes still more noticeable; and it is by these slow steps that our judgment arrives at full confidence. But before it reaches such completeness it passes through several lower degrees, and in all of them it is to be regarded as only a presumption or probability. So the gradation from probabilities to proofs is in many cases imperceptible, and large differences between these kinds of confidence are easier to perceive than small ones.