Expositor's Bible Commentary Commentary Luke 6:45

"The good man out of the good treasure of his heart bringeth forth that which is good; and the evil [man] out of the evil [treasure] bringeth forth that which is evil: for out of the abundance of the heart his mouth speaketh." — Luke 6:45 (ASV)

The thought of v.40 continues—like teacher, like student; like tree, like fruit. Throughout the preceding section and this one, the idea is that of consistency between source and product.