Jamieson, Fausset and Brown on Isaiah 6:10

10. Make ... fat--(Psa 119:17). "Render them the more hardened by thy warnings" [Maurer]. This effect is the fruit, not of the truth in itself, but of the corrupt state of their hearts, to which God here judicially gives them over (Is 63:17). Gesenius takes the imperatives as futures. "Proclaim truths, the result of which proclamation will be their becoming the more hardened" (Ro 1:28; Ep 4:18); but this does not so well as the former set forth God as designedly giving up sinners to judicial hardening (Ro 11:8; 2Th 2:11). In the first member of the sentence, the order is, the heart, ears, eyes; in the latter, the reverse order, the eyes, ears, heart. It is from the heart that corruption flows into the ears and eyes (Mr 7:21, 22); but through the eyes and ears healing reaches the heart (Ro 10:17), [Bengel]. (Jr 5:21; Eze 12:2; Zec 7:11; Ac 7:57; 2Ti 4:4). In Mt 13:15, the words are quoted in the indicative, "is waxed gross" (so the Septuagint), not the imperative, "make fat"; God's word as to the future is as certain as if it were already fulfilled. To see with one's eyes will not convince a will that is opposed to the truth (compare Joh 11:45, 46; 12:10, 11). "One must love divine things in order to understand them" [Pascal].

be healed--of their spiritual malady, sin (Is 1:6; Psa 103:3; Jr 17:14).