We collect a number of these concurrences, and having, by their perceived uniformity, reduced them into classes, form a general idea annexed to that part of the subject which stands as the antecedent. It must be admitted that a definition of any substance, that is anything that deserves to be called knowledge respecting it, will enable us to predict some of its future probable consequences, and that for this plain reason that definition is prediction under another name. But, though, when we have gained the idea of impenetrability as a general phenomenon of matter, we can predict some of the variations to which it leads, there are others which we cannot predict:Chapter r, in other words, we know none of these variations but such as we have actually remarked, added to an expectation that similar events will arise under similar circumstances, proportioned to the constancy with which they have been observed to take place in our past experience. Finding, as we do by repeated experiments, that material substances have the property of resistance, and that one substance in a state of rest, when struck upon by another, passes into a state of motion, we are still in want of more particular observation to enable us to predict the specific varieties that will follow from this collision, in each of the bodies. Enquire of a man who knows nothing more of matter than its general property of impenetrability what will be the result of one ball of matter impinging upon another, and you will soon find how little this general property can inform him of the particular laws of motion. We suppose him to know that motion will follow in to the second ball. But what quantity of motion will be communicated?
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