John Gill Commentary Mark 8:17

"And Jesus perceiving it saith unto them, Why reason ye, because ye have no bread? do ye not yet perceive, neither understand? have ye your heart hardened?" — Mark 8:17 (ASV)

And when Jesus knew it
As he did immediately, by his omniscience; for as he knew the thoughts and reasonings of the Scribes and Pharisees, (Matthew 9:4), so he did those of his own disciples:

he says to them, why reason you because you have no bread ?
or imagine that I have given you this caution on that account; or are distressed because this is your case, as if you should be reduced to great difficulties, by reason of your forgetfulness and negligence:

perceive you not yet, neither understand ?
the meaning of the parabolical expressions, which he had used them to; or his power in providing food for them, and supporting a great number of persons with very little food, of which they had some very late instances:

have you your heart yet hardened ?
as after the first miracle; see (Mark 6:52), for it might have been expected, that by a second miracle of the loaves, their understandings would have been more enlightened, and their faith increased, and that they would have relinquished their gross notions, their anxieties, doubts, and unbelief.