We do not causally connect every chance pair of successive events, but those only which have been repeatedly observed together.  The wonder is, then, that through oft-repeated observation of certain objects we come to believe that we know something about the behavior of other like objects, and the further behavior of these same ones.  From the fact that I have seen a given apple fall ten times to the ground, I infer that all the apples in the world do the same when loosened, instead of flying upward, which, in itself, is quite as thinkable; I infer further that this has always been the case, and will continue to be so to all eternity.  Where is the intermediate link between the proposition, “I have found that such an object has always been attended with such an effect,” and this other, “I foresee that other objects which are, in appearance, similar, will be attended with similar effects”?  This postulate, that the future will be like the past, and that like causes will have like effects, rests on a purely psychological basis.  In virtue of the laws of association the sight of an object or event vividly recalls the image of a second, often observed in connection with the former, and leads us involuntarily to expect its appearance anew.  The idea of causal connection is based on feeling (the feeling of inner determination to pass from one idea to a second), not upon insight; it is a product of the imagination, not of the understanding.  From the habitual perception of two events in connection (sunshine and heat) arises the mental determination to think of the second when we perceive the first, and, anticipating the senses, to count on its appearance.  It is now possible to state of what impression the idea of the causal nexus is the copy:  the impression on which it is based is the habitual transition from the idea of a thing to its customary attendant.  Hence the idea of causality has a purely subjective significance, not the objective one which we ascribe to it.  It is impossible to determine whether there is a real necessity of becoming corresponding to the felt necessity of thought.  In life we never doubt the fact, but for science our conviction of the uniformity of nature remains a merely probable (though a very highly probable) conviction.  Complete certainty is vouchsafed only by rational demonstration and immediate experience.  The necessary bond which we postulate between cause and effect can neither be demonstrated nor felt.

If all experiential reasonings depend on the idea of causality, and this has no other support than subjective mental habit, it follows that all knowledge of nature which goes beyond mere observed fact is not knowledge (neither demonstrative knowledge nor knowledge of fact), but belief.[1] The probability of our belief in the regularity of natural phenomena increases, indeed, with every new verification of the assumptions based thereon; but, as has been shown, it never rises to absolute certainty.  Nevertheless inferences from