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, though he knows himself to be perfectly secure from falling, by his experience of the solidity of the iron, which supports him; and though 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 can-not 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 inlivens 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.

But why need we seek for other instances, while the present subject of philosophical probabilities offers us so obvious an one, in the opposition betwixt the judgment and imagination arising from these effects of custom?  According to my system, all reasonings are nothing but the effects of custom; and custom has no influence, but by inlivening the imagination, and giving us a strong conception of any object.  It may, therefore, be concluded, that our judgment and imagination can never be contrary, and that custom cannot operate on the latter faculty after such a manner, as to render it opposite to the former.  This difficulty we can remove after no other manner, than by supposing the influence of general rules.  We shall afterwards take [Sect. 15.] notice of some general rules, by which we ought to regulate our judgment concerning causes and effects; and these rules are formed on the nature of our understanding, and on our experience of its operations in the judgments we form concerning objects.  By them we learn to distinguish the accidental circumstances from the efficacious causes; and when we find that an effect can be produced without the concurrence of any particular circumstance, we conclude that that circumstance makes not a part of the efficacious cause, however frequently conjoined with it.  But as this frequent conjunction necessity makes it have some effect on the imagination, in spite of the opposite conclusion from general rules, the opposition of these two principles produces a contrariety in our thoughts, and causes us to ascribe the one inference to our judgment, and the other to our imagination.  The general rule is attributed to our judgment; as being more extensive and constant.  The exception to the imagination, as being more capricious and uncertain.