A quick addendum to my earlier post: The Sermon on the Mount was directed to "the crowds," not to the Pharisees in particular. The Pharisees--which is to say, the precursors to Orthodox Judaism--were well aware of the existence of "thought sin" and hypocrisy among many claiming the title, and were trying to deal with it accordingly. Indeed, Yeshua's criticism in Matthew 23 reads very close to a similar criticism that the Pharisees leveled against their own membership in the Babylonian Talmud, tractate Sotah.
The idea that Judaism focuses or focused only on the external is based in an ignorance of Judaism's actual teachings.
Shalom.
It looks like Jesus’ “But I say” rule assumes that Torah, as interpreted by the Pharisees, covered acts and He was expanding the rule. However, since there was no “covering” for intentional acts some thought sin must have been intended.
Mat 5:20 For I say unto you, That except your righteousness shall exceed [the righteousness] of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.
Mat 5:21-22 Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
Mat 5:27-28 Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Job 31:1-4, I made a covenant with mine eyes; why then should I think upon a maid? For what portion of God [is there] from above? and [what] inheritance of the Almighty from on high? [Is] not destruction to the wicked? and a strange [punishment] to the workers of iniquity? Doth not he see my ways, and count all my steps?
“The Sermon on the Mount was directed to “the crowds,” not to the Pharisees in particular.”
When Jesus prefaced his arguments with, “You have heard it said”, from whom did the crowd hear such things?
“The idea that Judaism focuses or focused only on the external is based in an ignorance of Judaism’s actual teachings.”
Nearly all, if not all, of Jesus’ polemics against the Pharisees was because of their externalism.