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 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, 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 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.