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 it is 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 it is 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 with the essential, they have such an influence on the imagination, that even in the absence of the latter they carry us on to t-he 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 it is still certain, that custom takes the start, and gives a biass to the imagination.