If we admit that evil is an essential part of our being and the key to the interpretation of our life, we load ourselves down with a difficulty that has always proved burdensome in philosophies of religion.
 William James, The Varieties of Religious Experience (1902). copy citation

Context

“But as I closed our last hour with a general philosophical reflection on the healthy-minded way of taking life, I should like at this point to make another philosophical reflection upon it before turning to that heavier task. You will excuse the brief delay. If we admit that evil is an essential part of our being and the key to the interpretation of our life, we load ourselves down with a difficulty that has always proved burdensome in philosophies of religion. Theism, whenever it has erected itself into a systematic philosophy of the universe, has shown a reluctance to let God be anything less than All-in-All. In other words, philosophic theism has always shown a tendency to become pantheistic and monistic, and to consider the world as one unit of absolute fact;” source