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I add, as a third instance of this kind, that though our reasonings from proofs and from probabilities be considerably different from each other, yet the former species of reasoning often degenerates insensibly into the latter, by nothing but the multitude of connected arguments. 'Tis certain, that when an inference is drawn immediately from an object, without any intermediate cause or effect, the conviction is much stronger, and the persuasion more lively, than when the imagination is carried through a long chain of connected arguments, however infallible the connexion of each link may be esteemed. 'Tis from the original impression that the vivacity of all the ideas is derived, by means of the customary transition of the imagination; and 'tis evident this vivacity must gradually decay in proportion to the distance, and must lose somewhat in each transition. Sometimes this distance has a greater influence than even contrary experiments would have; and a man may receive a more lively conviction from a probable reasoning which is close and immediate, than from a long chain of consequences, though just and conclusive in each part. Nay, 'tis seldom such reasonings produce any conviction; and one[Pg 194] must have a very strong and firm imagination to preserve the evidence to the end, where it passes through so many stages.

But here it may not be amiss to remark a very curious phenomenon which the present subject suggests to us. 'Tis evident there is no point of ancient history, of which we can have any assurance, but by passing through many millions of causes and effects, and through a chain of arguments of almost an immeasurable length. Before the knowledge of the fact could come to the first historian, it must be conveyed through many mouths; and after it is committed to writing, each new copy is a new object, of which the connexion with the foregoing is known only by experience and observation. Perhaps therefore it may be concluded, from the precedent reasoning, that the evidence of all ancient history must now be lost, or at least will be lost in time, as the chain of causes increases, and runs on to a greater length. But as it seems contrary to common sense to think, that if the republic of letters and the art of printing continue on the same footing as at present, our posterity, even after a thousand ages, can ever doubt if there has been such a man as Julius Csar; this may be considered as an objection to the present system. If belief consisted only in a certain vivacity, conveyed from an original impression, it would decay by the length of the transition, and must at last be utterly extinguished. And, vice versa, if belief, on some occasions, be not capable of such an extinction, it must be something different from that vivacity.

Before I answer this objection I shall observe, that from this topic there has been borrowed a very celebrated argument against the Christian Religion; but[Pg 195] with this difference, that the connexion betwixt each link of the chain in human testimony has been there supposed not to go beyond probability, and to be liable to a degree of doubt and uncertainty. And indeed it must be confessed, that in this manner of considering the subject (which, however, is not a true one), there is no history or tradition but what must in the end lose all its force and evidence. Every new probability diminishes the original conviction; and, however great that conviction may be supposed, 'tis impossible it can subsist under such reiterated diminutions. This is true in general, though we shall find afterwards,[9] that there is one very memorable exception, which is of vast consequence in the present subject of the understanding.

Meanwhile, to give a solution of the preceding objection upon the supposition that historical evidence amounts at first to an entire proof, let us consider, that, though the links are innumerable that connect any original fact with the present impression, which is the foundation of belief, yet they are all of the same kind, and depend on the fidelity of printers and copyists. One edition passes into another, and that into a third, and so on, till we come to that volume we peruse at present. There is no variation in the steps. After we know one, we know all of them; and after we have made one, we can have no scruple as to the rest. This circumstance alone preserves the evidence of history, and will perpetuate the memory of the present age to the latest posterity. If all the long chain of causes and effects, which connect any past event with any volume of history, were composed of parts different[Pg 196] from each other, and which 'twere necessary for the mind distinctly to conceive, 'tis impossible we should preserve to the end any belief or evidence. But as most of these proofs are perfectly resembling, the mind runs easily along them, jumps from one part to another with facility, and forms but a confused and general notion of each link. By this means, a long chain of argument has as little effect in diminishing the original vivacity, as a much shorter would have if composed of parts which were different from each other, and of which each required a distinct consideration.

A fourth unphilosophical species of probability is that derived from general rules, which we rashly form to ourselves, and which are the source of what we properly call prejudice. An Irishman cannot have wit, and a Frenchman cannot have solidity; for which reason, though the conversation of the former in any instance be visibly very agreeable, and of the latter very judicious, we have entertained such a prejudice against them, that they must be dunces or fops in spite of sense and reason. Human nature is very subject to errors of this kind, and perhaps this nation as much as any other.

Should it be demanded why men form general rules, and allow them to influence their judgment, even contrary to present observation and experience, I should reply, that in my opinion it proceeds from those very principles on which all judgments concerning causes and effects depend. Our judgments concerning cause and effect are derived from habit and experience; and when we have been accustomed to see one object united to another, our imagination passes from the first to the second by a natural transition, which precedes reflection,[Pg 197] and which cannot be prevented by it. Now, 'tis the nature of custom not only to operate with its full force, when objects are presented that are exactly the same with those to which we have been accustomed, but also to operate in an inferior degree when we discover such as are similar; and though the habit loses somewhat of its force by every difference, yet 'tis seldom entirely destroyed where any considerable circumstances remain the same. A man who has contracted a custom of eating fruit by the use of pears or peaches, will satisfy himself with melons where he cannot find his favourite fruit; as one, who has become a drunkard by the use of red wines, will be carried almost with the same violence to white, if presented to him. From this principle I have accounted for that species of probability, derived from analogy, where we transfer our experience in past instances to objects which are resembling, but are not exactly the same with those concerning which we have had experience. In proportion as the resemblance decays, the probability diminishes, but still has some force as long as there remain any traces of the resemblance.

This observation we may carry farther, and may remark, that though custom be the foundation of all our judgments, yet sometimes it has an effect on the imagination in opposition to the judgment, and produces a contrariety in our sentiments concerning the same object. I explain myself. In almost all kinds of causes there is a complication of circumstances, of which some are essential, and others superfluous; some are absolutely requisite to the production of the effect, and others are only conjoined by accident. Now we may observe, that when these superfluous circumstances are numerous and remarkable, and frequently conjoined[Pg 198] with the essential, they have such an influence on the imagination, that even in the absence of the latter they carry us on to the conception of the usual effect, and give to that conception a force and vivacity which make it superior to the mere fictions of the fancy. We may correct this propensity by a reflection on the nature of those circumstances; but 'tis still certain, that custom takes the start, and gives a bias to the imagination.