No disease of the imagination is so difficult to cure, as that which is complicated with the dread of guilt: fancy and conscience then act interchangeably upon us, and so often shift their places, that the illusions of one are not distinguished from the dictates of the other.

Samuel Johnson (1709-1784) English writer, lexicographer, critic

The History of Rasselas, Prince of Abissinia, ch. 46 (1759)
    (Source)

Sometimes attributed to E. M. Forster, as he transcribed the words in his Commonplace Book.