This world is a comedy to those that think; a tragedy to those that feel.

Horace Walpole (1717-1797) English novelist, letter writer

Letter to Anne, Countess of Upper Ossory (16 Aug 1776)
    (Source)

Walpole frequently used used this phrase or variants in letters (and in fact prefaces this quote with "I have often said ..."). Another example is an earlier letter to Horace Mann (31 Dec 1769):

I have often said, and oftener think, that this world is a comedy to those that think, a tragedy to those that feel -- a solution of why Democritus laughed and Heraclitus wept.

It may be derived from an (unsourced) similar quote attributed Jean de La Bruyère: "Life is a tragedy for those who feel, and a comedy for those who think".