4 So then those words of the Lord will not disturb you, when He says, “Every tree is known by his own fruit. Do men gather grapes of thorns, and figs of thistles?” The Scribes and Pharisees of the Jews therefore were thorns and thistles, and notwithstanding, “what they say do, but do ye not after their works.” So then the grape is gathered from thorns, and the fig from thistles, as He has given you to understand according to the method I have just laid down. For so sometimes in the vineyard's thorny hedge, the vines get entangled, and clusters of grapes hang from the brambles. You had no sooner heard the name of thorns, than you were on the point of disregarding the grape. But seek for the root of the thorns, and you will see where to find it. Follow too the root of the hanging cluster, and you will see where to find it. So understand that the one refers to the Pharisee's heart, the other to Moses' seat.
Source: Sermons on Selected Lessons of the New Testament (New Advent)