Quoted from Isaiah 6:9, 10. Some texts such as the Byzantine Text from which the KJV was translated, read “of sins” at the end of the verse, but scholars are now aware that this is an explanatory gloss that worked its way into the text.
“so that…” For this quotation from Isaiah and the purpose of parables, see commentary on Matthew 13:13. The “so that” is the Greek word hina plus the verb in the subjunctive mood, which shows this to be a purpose-result clause: see commentary on Matthew 2:15, “resulting in…what was spoken being fulfilled.” To fully understand this passage, we must see how Matthew’s record portrays the human side of the events, John’s the spiritual side, and Mark and Luke’s records combine the two into one.
The subjunctive mood in the verse makes it somewhat difficult to understand and translate. The subjunctive is forced by the use of “so that” (Greek, hina, #2443), so the verb does not have to be translated like a subjunctive, which would be “they may see and may not perceive,” but the use of “lest” in the last phrase makes a subjunctive translation awkward. Wicked people look and look without seeing, and hear and hear without understanding.