John Calvin Commentary Luke 6:8

"But he knew their thoughts; and he said to the man that had his hand withered, Rise up, and stand forth in the midst. And he arose and stood forth." — Luke 6:8 (ASV)

But he knew their thoughts. If Matthew states the truth, they had openly declared by their language what was in their minds; and therefore Christ replies not to their secret thoughts, but to express words.

But both may be true: that they spoke plainly, and yet that Christ discerned their secret thoughts. For they did not openly avow their designs, and Matthew himself tells us that their question was intended to take Christ by surprise. Consequently, Luke means nothing more than that Christ was aware of their insidious designs, though not expressed in words.