Luke 10:29

θέλων δικαιῶσαι ἑαυτόν. Desiring to justify himself “before men”—a thing which the Pharisees were ever prone to do, Luke 16:15. He felt that Christ’s answer involved a censure and therefore wished to justify his question.

τίς ἐστίν μου πλησίον; No doubt the meaning is who is my neighbour? but as πλησίον is an adv. the omission of the article is unusual and not easily explained. He wants his moral duties to be labelled and defined with the Talmudic precision to which ceremonial duties had been reduced.