Irony is objective, that is, intended for another; but humour is subjective, that is, it primarily exists only for one's own self.
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

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Author Arthur Schopenhauer
Source The World as Will and Representation
Topic irony self-knowledge
Date 1819
Language English
Reference
Note Translated by R. B. Haldane and J. Kemp
Weblink http://www.gutenberg.org/files/40097/40097-h/40097-h.html

Context

“It might be called the double counterpoint of irony. Explanations such as “Humour is the interpenetration of the finite and the infinite” express nothing more than the entire incapacity for thought of those who are satisfied with such empty phrases. Irony is objective, that is, intended for another; but humour is subjective, that is, it primarily exists only for one's own self. Accordingly we find the masterpieces of irony among the ancients, but those of humour among the moderns. For, more closely considered, humour depends upon a subjective, yet serious and sublime mood, which is involuntarily in conflict with a common external world very different from itself, which it cannot escape from and to which it will not give itself up;” source