We see, in nature, one event followed by another. The fall of a spark on gunpowder, for example, followed by the deflagration of the gunpowder; and, by a peculiar tendency of our constitution, which we must take for granted, whatever be our theory of power, we believe, that as long as all the circumstances continue the same, the sequence of events will continue the same; that the deflagration of gunpowder, for example, will be the invariable consequence of the fall of a spark on it;--in other words, we believe the gunpowder to be susceptible of deflagration on the application of a spark,--and a spark to have the power of deflagrating gunpowder. There is nothing more, then, understood in the trains of events, however regular, than the regular order of antecedents and consequents which compose the train; and between which, if any thing else existed, it would itself be a part of the train. All that we mean, when we ascribe to one substance a susceptibility of being affected by another substance, is, that a certain change will uniformly take place in it when that other is present;--all that we mean, in like manner when we ascribe to one substance a power of affecting another substance, is, that, when it is present a certain change will uniformly take place in that other substance. Power, in short, is significant not of any thing different from the invariable antecedent itself, but of the mere invariableness of the order of its appearance in reference to some invariable consequent,--the invariable antecedent being denominated a _cause_, the invariable consequent an _effect_. To say, that water has the power of dissolving salt, and to say, that salt will always melt when water is poured upon it, are to say precisely the same thing;--there is nothing in the one proposition, which is not exactly, and to the same extent, enunciated in the other.