From Why Can’t We Be Good?
The idea of the One God leads the pupil to the idea of the human heart – that at one and the same time the human heart wishes both to receive and to reject what is offered to it. Here, in front of this contradiction, the pupil – we, ourselves – confront the great idea of man’s “two hearts,” his two natures, or, as it is called in the Hebraic formulation, the yetzer tov and the yetzer ha-ra, the good impulse and the inclination toward evil. The Torah calls us to inform both of our “two hearts” about the oneness and the greatness of God. It calls us to serve God with both the good impulse and the inclination toward evil, just as, at the root of his creation of the world, the good and the evil, the angel and the devil, were created to further the action in the world of the One-Without-a-Name, the God beyond God – what the hidden tradition-without-a-name refers to as the great In-Itself, the En-Sof.
The two hearts of man must be penetrated and reconciled by the truth of God. As we are now, one impulse affirms and the other impulse denies. The practice of the teaching will involve bringing these two movements into harmony – through submission to the One-Without-a-Name within ourselves.
God is One – yes, but also, and of highest importance: God is Oneness. God is Unity. The pupil is obliged to understand what this means. The pupil – man – does not understand this idea – because, in order to understand it, he must himself be on the way to unity within his own being. Without a taste of the experience of unity within himself, the idea remains only a noble ideal, a noble concept which the good impulse with him accepts – like an “obedient” follower, a “good Jew.” But it is not enough to be a “good Jew.” Or rather – it is impossible to be a “good Jew,” a good man, without including in one’s consciousness all that resists the impulse toward Good.