Because seeing they see not, &c. i.e. they see not as they might, and ought to do, by shutting their eyes against the lights given them.

Therefore do I speak to them in parables: because seeing they see not, &c. This passage, by which the prophet Isaias (vi. 9.) was ordered to foretell the obstinate blindness of the Jews, in refusing to receive and believe in their Messias, is cited six times in the New Testament; to wit, here in St. Matthew, also Mark iv. 14[12?]; Luke viii. 10; John xii. 40; Acts xxviii. 26; and Romans xi. 8. In all these places we must detest the false interpretation of those who, not without heresy and blasphemy, would have God to be the author and cause of sin. When it is said, (Isaias vi. 9.) blind the heart of this people, &c. the prophet is only commanded to foretell their blindness, of which, by their wilful obstinacy, they were the true cause. And when we read in St. Mark, that to those that are without, all things are done in parables, that seeing they may see, and not see, &c. the word that does not signify the cause, nor the end, but only the event, and the consequence of what would happen by the wilful blindness of the Jews, and by the just permission of God. St. Matthew here expounds to us the words of the prophet, by which it clearly appears that they were the cause of their own blindness; and that, by their obstinacy, they had made themselves unworthy of particular lights from God. For the heart of this people (ver. 18.[15.?]) is grown gross ... and their eyes they have shut, &c. The Jews therefore shut their own eyes, hardened their own hearts, which God only permitted. See Romans ix. 18. &c. (Witham)

If this blindness were natural, then indeed I would have opened their eyes to see and understand, but since this blindness is voluntary, he says, that seeing they see not, and hearing, they hear not; i.e. they have seen me cast out devils, and they said, in Beelzebub he casteth out devils; they heard I drew all to God, and they say, this man cometh not from God. Since, therefore, they assert the very contrary to what they both see and hear, the gift of seeing and hearing me shall be taken away from them.