Of the numerous antecedents and consequents, forming the matter of our experience, some are constant, some are not. Of course the strength of the association follows the frequency. The crow is seen flying as frequently from east to west, as from west to east; from north to south, as from south to north; there is, therefore, no association between the flight of the crow and any particular direction. Not so with the motion of a stone let go in the air: that takes one direction constantly. The order of antecedent and consequent is here invariable. The association of the ideas, therefore, is fixed and inseparable. I can no more have the idea of a stone let go in the air, and not have the idea of its dropping to the {364} ground, than I can have the idea of the stone, and not have it, at the same time.[102] [Editor's footnote 102: The theory maintained so powerfully and with such high intellectual resources by the author, that Belief is but an inseparable association, will be examined at length in a note at the end of the chapter. Meanwhile let it be remarked, that the case of supposed inseparable association given in this passage, requires to be qualified in the statement. We cannot, indeed, think of a stone let go in the air, without having the idea of its falling; but this association is not so strictly inseparable as to disable us from having the contrary idea. There are analogies in our experience which enable us without difficulty to form the imagination of a stone suspended in the air. The case appears to be one in which we can conceive both opposites, falling and not falling; the incompatible images not, of course, combining, but alternating in the mind. Which of the two carries belief with it, depends on what is termed Evidence.--_Ed._] Where the sequence of two events is merely casual, it passes speedily away from the mind; because it is not associated with the idea of any thing in which we are interested. The things in which we are interested, are the immediate antecedents of our pleasures and pains, and the ideas of them are all inseparably associated with constant conjunctions. The association of the ideas of a constant antecedent and consequent, therefore, has both causes of strength, the interesting nature of the ideas, and the frequency of conjunction, both at their greatest height. It follows, that it should be the most potent and inseparable of all the combinations in the mind of man.