Jamieson, Fausset and Brown on Luke 18:20

20. Thou knowest, &c.--Matthew (Mt 19:17) is more complete here: "but if thou wilt enter into life, keep the commandments. He saith unto him, Which--as if he had said, Point me out one of them which I have not kept?--"Jesus said, Thou shalt," &c. (Mt 19:17, 18). Our Lord purposely confines Himself to the second table, which He would consider easy to keep, enumerating them all--for in Mark (Mr 10:19), "Defraud not" stands for the tenth (else the eighth is twice repeated). In Matthew (Mt 19:19) the sum of this second table of the law is added, "Thou shalt love thy neighbor as thyself," as if to see if he would venture to say he had kept that.