One general description of madness, it seems to us, might be found in the statement that madness is a preference for the symbol over that which it represents....Money, for example, is a symbol; it symbolises wine and horses and beautiful vesture and high houses, the great cities of the world and the quiet tent by the river. The miser is a madman, because he prefers money to all these things; because he prefers the symbol to the reality....

...The essential of idolatry is the same. Idolatry exists wherever the thing that originally gave us happiness becomes at last more important than happiness itself. Drunkenness, for example, may be fairly described as an engrossing hobby. And drunkenness is, when really comprehended in its inward and psychological reality, a typical example of idolatry. Essential intemperance begins at the point where the one incidental form of pleasure, which comes from a certain article of consumption, becomes more important than all the vast universe of natural pleasures, which it finally destroys....

...This is idolatry: the preference for the incidental good over the eternal good which it symbolises. It is the employment of one example of the everlasting goodness to confound the validity of a thousand other examples. It is the elementary mathematical and moral heresy that the part is greater than the whole.

-"Lunacy and Letters" (1901)
Found in Lunacy and Letters (collection of essays published posthumously in 1958)