John Gill Commentary Mark 4:12

"that seeing they may see, and not perceive; and hearing they may hear, and not understand; lest haply they should turn again, and it should be forgiven them." — Mark 4:12 (ASV)

That seeing they may see
Which is the end and reason of his speaking to them in parables. The passage referred to is in (Isaiah 6:9Isaiah 6:10) . (See Gill on Matthew 13:14). (See Gill on Matthew 13:15).