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?
What result will follow upon the collision, in the impelling ball? Will it continue to move in the same direction? Will it recoil in the opposite direction? Will it fly off obliquely; or will it subside into a state of rest? All these events will be found equally probable by him whom a series of observations upon the past has not instructed as to what he is to expect from the future.
From these remarks we may sufficiently collect what is the species of knowledge we possess respecting the laws of the material universe. No experiments we are able to make, no reasonings we are able to deduce, can ever instruct us in the principle of causation, or show us for what reason it is that one event has, in every instance in which it has been known to occur, been the precursor of another event of a given description. Yet this observation does not, in the slightest degree, invalidate our inference from one event to another, or affect the operations of moral prudence and expectation.
The nature of the human mind is such as to oblige us, after having seen two events perpetually conjoined, to pass, as soon as one of them occurs, to the recollection of the other: and, in cases where this transition never misleads us, but the ideal succession is always found to be an exact copy of the future event, it is impossible that this species of foresight should not be converted into a general foundation of inference and reasoning.
We cannot take a single step upon this subject which does not partake of the species of operation we denominate abstraction. Till we have been led to consider the rising of the sun tomorrow as an incident of the same species as its rising today, we cannot deduce from it similar consequences. It is the business of science to carry this talk of generalization to its furthest extent, and to reduce the diversified events of the universe to a small number of original principles.
Let us proceed to apply these reasonings concerning matter to the illustration of the theory of mind. Is it possible in this latter theory, as in the former subject, to discover any general principles? Can intellect be made a topic of science? Are we able to reduce the multiplied phenomena of mind to any certain standard of reasoning? If the affirmative of these questions be conceded, the inevitable consequence appears to be that mind, as well as matter, exhibits a constant conjunction of events, and furnishes all the ground that any subject will afford for an opinion of necessity. It is of no importance that we cannot see the ground of that necessity, or imagine how sensations, pleasurable or painful, when presented to the mind of a percipient being, are able to generate volition and animal motion;Chapter or, if there be any truth in the above statement, we are equally incapable of perceiving a ground of connection between any two events in the material universe, the common and received opinion, that we do perceive such ground of connection, being, in reality, nothing more than a vulgar prejudice.