A Treatise of Human Nature

Book I, Part III: Section XIII (cont.)

David Hume

Section XIII. Of unphilosophical probability. (cont.)

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 tho' 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 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, tho' 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, 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 tho' 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 tho' 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 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.

To illustrate this by a familiar instance, let us consider the case of a man, who being hung out from a high tower in a cage of iron cannot forbear trembling, when he surveys the precipice below him, tho' he knows himself to be perfectly secure from falling, by his experience of the solidity of the iron, which supports him; and tho' the ideas of fall and descent, and harm and death, be derived solely from custom and experience. The same custom goes beyond the instances, from which it is derived, and to which it perfectly corresponds; and influences his ideas of such objects as are in some respect resembling, but fall not precisely under the same rule. The circumstances of depth and descent strike so strongly upon him, that their influence cannot be destroyed by the contrary circumstances of support and solidity, which ought to give him a perfect security. His imagination runs away with its object, and excites a passion proportioned to it. That passion returns back upon the imagination and enlivens the idea; which lively idea has a new influence on the passion, and in its turn augments its force and violence; and both his fancy and affections, thus mutually supporting each other, cause the whole to have a very great influence upon him.