I know that thou canst do everything - This is said by Job in view of what had been declared by the Almighty in the previous chapters. It is an acknowledgment that God was omnipotent, and that man ought to be submissive, under the putting forth of his infinite power. One great object of the address of the Almighty was to convince Job of his majesty, and that object was fully accomplished.
And that no thought - No purpose or plan of thine. God was able to execute all his designs.
Can be withholden from thee - Margin, "or, of thine can be hindered." Literally, "cut off" - בצר bâtsar. The word, however, means also "to cut off access to," and then to prevent, hinder, restrain. This is its meaning here; so Genesis 11:6, "Nothing will be restrained (יבצר yibâtsar) from them, which they have imagined to do."

I know that thou canst do every thing - Thy power is unlimited; thy wisdom infinite.

I know that thou canst do every [thing], and [that] no (a) thought can be withholden from thee.
(a) No thought so secret but you see it, nor anything that you think but that you can bring it to pass.

I know that thou canst do every thing,.... As the works of creation, and the sustentation of them, show; so the Targum,
"thou sustainest all things,''
and can manage, every creature made by him, even such as were not tractable by men, such as behemoth and leviathan, the creatures last instanced in; and was able to abase and bring low the proud, which Job could not do; and could also save him by his right hand, and bring him out of his low estate in which he was, and raise him to great prosperity again, which Job always despaired of till now; and though he had a theoretical knowledge of the omnipotence of God before, see Job 9:4; yet not a practical experimental knowledge of it; at least not to such a degree as he now had, working upon his heart, bowing his will, and bringing him to a resignation to the will of God; he not only knew he could do all things, but that he had a right to do what he pleased; and that whatever he did he did well and wisely, and in a righteous manner, of which before he seemed to have some doubt. And that no thought can be withholden from thee; either no thought of men, good or bad, of God or of themselves, and so is an acknowledgment of the omniscience of God, and may be an appeal to that; that God, who knows the secrets of men's hearts, knew what thoughts Job now had of God; of the wisdom, righteousness, and goodness of God in the dispensations of his providence, different from what he had before; see John 21:17; or rather it may be understood of every thought of God's heart, of every secret purpose and wise counsel of his; which, as they are all well known to him, and cannot be withheld from having effect, or the performance of them hindered, Job now saw and was fully assured that all that had befallen him was according to the sovereign and inscrutable purposes of God, and according to the wise counsels of his will; he knew that not only God could do everything, but that he also did whatever he pleased.

JOB'S PENITENT REPLY. (Job 42:1-6)
In the first clause he owns God to be omnipotent over nature, as contrasted with his own feebleness, which God had proved (Job 40:15; Job 41:34); in the second, that God is supremely just (which, in order to be governor of the world, He must needs be) in all His dealings, as contrasted with his own vileness (Job 42:6), and incompetence to deal with the wicked as a just judge (Job 40:8-14).
thought--"purpose," as in Job 17:11; but it is usually applied to evil devices (Job 21:27; Psalm 10:2): the ambiguous word is designedly chosen to express that, while to Job's finite view, God's plans seem bad, to the All-wise One they continue unhindered in their development, and will at last be seen to be as good as they are infinitely wise. No evil can emanate from the Parent of good (James 1:13, James 1:17); but it is His prerogative to overrule evil to good.

Thou canst, &c. - Job here subscribes to God's unlimited power, knowledge and dominion, to prove which was the scope of God's discourse out of the whirlwind. And his judgment being convinced of these, his conscience also was convinced, of his own folly in speaking so irreverently concerning him. No thought can be withholden from thee - No thought of ours can be withholden from thy knowledge. And there is no thought of thine, which thou canst be hindered from bringing into execution.