When happiness is actually in possession, the thought of evil can no more acquire the feeling of reality than the thought of good can gain reality when melancholy rules.
 William James, The Varieties of Religious Experience (1902). copy citation

Context

“and although, when thus nakedly stated, this might seem a difficult feat to perform for one who is intellectually sincere with himself and honest about facts, a little reflection shows that the situation is too complex to lie open to so simple a criticism. In the first place, happiness, like every other emotional state, has blindness and insensibility to opposing facts given it as its instinctive weapon for self-protection against disturbance. When happiness is actually in possession, the thought of evil can no more acquire the feeling of reality than the thought of good can gain reality when melancholy rules. To the man actively happy, from whatever cause, evil simply cannot then and there be believed in. He must ignore it; and to the bystander he may then seem perversely to shut his eyes to it and hush it up.” source