The literary relationship (if any) between Matthew and Luke continues to be debated, and so the relationship between the Sermon on the Mount and the Sermon on the Plain continues to be debated (with implications for how one reads both Matthew and Luke). This essay argues that the confluence of several facts – (1) obviously Matthean redaction is missing from Luke 6:21–49; (2) the more we subtract Matthean features from Matthew 5–7, the closer we get to the Sermon on the Plain; (3) 1 Clement 13:2, which seems to be independent of the synoptics, is closer at points to Luke 6:27–42 than it is to the Matthean parallels; (4) intertextual ties to Leviticus 19 are clustered in Luke 6:27–42 but separated in Matthew 5 and 7; and (5) in a number of instances, words and expressions in Luke 6:21–47 seem, by the standard tools of our trade, to be more original than their Matthean counterparts – adds up to a strong case that the Sermon on the Mount was not the source for the Sermon on the Plain. Luke is not a reader or an interpreter of Matthew’s Gospel. The data rather support the Q hypothesis or the theory that Matthew was a reader of Luke.