Man is the only animal that laughs and weeps, for he is the only animal that is struck with the difference between what things are and what they ought to be. We weep at what exceeds our expectations in serious matters; we laugh at what disappoints our expectations in trifles. We shed tears from sympathy with real and necessary distress; as we burst into laughter from want of sympathy with that which is unreasonable and unnecessary. Tears are the natural and involuntary response of the mind overcome by some sudden and violent emotions. Laughter is the same sort of convulsive and involuntary movement, occasioned by mere surprise or contrast.

The serious is the stress which the mind lays upon the expectations of a given order of events and the weight attached to them. When this stress is increased beyond its usual intensity and strains the feelings by the violent opposition of good and bad, it becomes the tragic. The ludicrous is the unexpected relaxing of this stress below its usual intensity, by an abrupt transposition of ideas that takes the mind by surprise and startles it into lively sense of pleasure.