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There is a principle in our nature which makes us dissatisfied with unexplained and unconnected facts; which leads us to theorize all the particulars of our knowledge, or to form in our own minds some system of causes sufficient to explain or produce the effects which we see ; and which teaches us to believe or disbelieve in the truth of any system which may be presented to us, just as it appears adequate or inadequate to afford that explanation of which we are in pursuit. We have an intuitive perception that the appearances of Nature are connected by the relation of cause and effect; and we have also an instinctive desire to classify and arrange the seemingly confused mass of facts with which we are surrounded, according to this distinguishing relationship. From these principles have proceeded all the theories which were ever formed by man. But these principles alone can never make a true theory: They teach us to theorize; but experience is necessary in order to theorize justly. We must be acquainted with the ordinary operations of causes, before we can combine them into a theory which will satisfy the mind. But when we are convinced of the real existence of a cause in nature, and when we find that a class of physical facts is explained by the
supposition of this cause, and tallies exactly with its ordinary operation, we resist both reason and instinct when we resist the conviction that this class of facts does result from this
On this process of reasoning is grounded our conviction that the various phenomena of the heavenly bodies are results from the principle or law of gravitation. That great master of theories, Adam Smith, has given a most appropriate and beautiful illustration of this principle, in his “ History of Astronomy." He has there shown, how the speculative system was always accommodated to the phenomena which had been observed; and how, on each new discovery in point of fact, a corresponding change necessarily took place in the form of the system.
There is another process of reasoning, differing somewhat from that which has been described, yet closely allied to it; by which, instead of ascending from effects to a cause, descend from a cause to effects. When we are once convinced of the existence of a cause, and are acquainted with its ordinary mode of operation, we are prepared to give a certain degree of credit to a history of other effects attributed to it, provided we can trace the connexion between them. As an illustration of this, I thall suppose, that the steam-engine, and the application of it to the movement of vessels, was known in China in the days of Archimedes; and that a foolish lying traveller had found his way from Sicily to China, and had there seen an exhibition of a steam boat, and
had been admitted to examine the mechanical apparatus of it,--and, upon his return home, had amongst many palpable fables, related the true particulars of this exhibition,—what feeling would this relation have probably excited in his audience? The fact itself was a strange one, and different in appearance from any thing with which they were acquainted : It was also associated with other stories that seemed to have falsehood stamped on the very face of them. What means, then, had the hearers of distinguishing the true from the false? Some of the rabble might probably give a stupid and wondering kind of credit to the whole ; whilst the judicious but unscientific hearers would reject the whole. Now, supposing that the relation had come to the ears of Archimedes, and that he had sent for the man and interrogated him; and, from his unorderly and unscientific, but accurate specification of boilers, and cylinders, and pipes, and furnaces, and wheels, had drawn out the mechanical theory of the steamboat, -he might have told his friends, “The traveller may be d liar; but this is a truth. I have a stronger evidence for it than his testimony or the testimony of any man: It is a truth in the nature of things. The effect which the man has described is the legitimate and certain result of the apparatus which he has described. If he has fabricated this account, he must be a great philosopher. At all events, his narration is founded on an unquestionable general truth.” Had the traveller committed an error in his specification, that defect would have op
erated as an obstacle to the conviction of Archimedes; because, where the facts which are testified constitute the parts of a system, they must in order to produce conviction, be viewed in their relation to one another and in their combined bearing on the general result. Unless they are thus viewed, they are not seen as they really exist,—they do not hold their proper ground. A single detached pipe or boiler or valve could not produce the effects of the steam-engine ; and a man who knows no more about it than that it contains such a detached part, may very well laugh at the effects related of the whole machine; but, in truth, the fault lies in his own ignorance of the subject.
But these two processes of reasoning which have been described, are not exclusively applied to physical causes and effects : We reason precisely in the same way with regard to men and their actions. When the history of a man's life is presented to us, we naturally theorize upon it; and from a comparison of the different facts contained in it, we arrive at a conviction that he was actuated by ambition, avarice, benevolence, or some other principle. know that these principles exist, and we know also their ordinary mode of operation : When, therefore, we see the operation, we refer it to the cause which best explains it. In this manner we arrange the characters with which we are acquainted under certain classes; and we anticipate the conduct of our friends when they come to be placed in certain circumstances; and when we are at a distance from any