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.
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.