That thy benefit might not be by constraint. This is drawn from the general rule, that no sacrifices are acceptable to God but those which are freely offered. Paul speaks of almsgiving in the same manner. (2-Corinthians 9:7.) To agathon is here put for "acts of kindness," and willingness is contrasted with constraint, when there is no other opportunity of putting to the test a generous and cheerful act of the will; for that duty which is generously performed, and not through influence exercised by others, is alone entitled to full praise. It is also worthy of observation, that Paul, while he acknowledges that Onesimus was to blame in past time, affirms that he is changed; and lest Philemon should have any doubt that his slave returns to him with a new disposition and different conduct, Paul says that he has made full trial of his repentance by personal knowledge.

But without thy mind would I do nothing - Nothing in the matter referred to. He would not retain Onesimus in his service, much as he needed his assistance, without the cordial consent of Philemon. He would not give him occasion for hard feeling or complaint, as if Paul had induced him to leave his master, or as if he persuaded him to remain with him when he wished to return - or as if he kept him away from him when he owed him or had wronged him. All that is said here is entirely consistent with the supposition that Onesimus was disposed to return to his master, and with the supposition that Paul did not compel or urge him to do it. For it is probable that if Onesimus had proposed to return, it would have been easy for Paul to have retained him with him. He might have represented his own want of a friend. He might have appealed to his gratitude on account of his efforts for his conversion.
He might have shown him that he was under no moral obligation to go back. He might have refused to give him this letter, and might have so represented to him the dangers of the way, and the probability of a harsh reception, as effectually to have dissuaded him from such a purpose. But, in that case, it is clear that this might have caused hard feeling in the bosom of Philemon, and rather than do that he preferred to let him return to his master, and to plead for him that he might have a kind reception. It is, therefore, by no means necessary to suppose that Paul felt that Onesimus was under obligation to return, or that he was disposed to compel him, or that Onesimus was not inclined to return voluntarily; but all the cirumstances of the case are met by the supposition that, if Paul retained him, Philemon might conceive that he had injured him. Suppose, as seems to have been the case, that Onesimus "owed" Philemon Plm 1:18, and then suppose that Paul had chosen to retain him with himself, and had dissuaded him from returning to him, would not Philemon have had reason to complain of it?
There was, therefore, on every account, great propriety in his saying that he did not wish to use any influence over him to retain him with him when he purposed to return to Colosse, and that he felt that it would be wrong for him to keep him, much as he needed him, without the consent of Philemon. Nor is it necessary, by what is said here, to suppose that Onesimus was a slave, and that Paul believed that Philemon had a right to him and to his services as such. All that he says here would be met by the supposition that he was a hired servant, and would be in fact equally proper even on the supposition that he was an apprentice. In either case, he would feel that he gave just ground of complaint on the part of Philemon if, when Onesimus desired to return, he used any influence to dissuade him from it, and to retain him with himself. It would have been a violation of the rule requiring us to do to others as we would wish them to do unto us, and Paul therefore felt unwilling, much as he needed the services of Onesimus, to make use of any influence to retain him with him without the consent of his master.
That thy benefit - The favor which I might receive from thee by having the services of Onesimus. If Onesimus should remain with him and assist him, he would feel that the benefit which would be conferred by his services would be in fact bestowed by Philemon, for he had a right to the service of Onesimus, and, while Paul enjoyed it, he would be deprived of it. The word rendered "benefit" here - ἀγαθόν agathon - means good, and the sense is, "the good which you would do me;" to wit, by the service of Onesimus.
Should not be as it were of necessity - As it would be it Paul should detain Onesimus with him without affording Philemon an opportunity of expressing his assent. Paul would even then have felt that he was in fact receiving a "good" at the expense of Philemon, but it would not be a voluntary favor on his part.
But willingly - As it would be if he had given his consent that Onesimus should remain with him.

That thy benefit should not be as it were of necessity - If the apostle had kept Onesimus in his service, and written to Philemon to forgive him and permit him to stay, to this it is probable he would have agreed; but the benefit thus conceded might have lost much of its real worth by the consideration that, had he been at Colosse, Philemon would not have sent him to Rome; but, being there and in the apostle's service, he could not with propriety order him home: thus the benefit to the apostle would have appeared to have been of necessity. The apostle, therefore, by sending him back again, gave Philemon the opportunity to do all as if self-moved to it. This is a very delicate touch.

But without thy mind would I do nothing; that thy benefit should not be as it were of (e) necessity, but willingly.
(e) That you might not seem to have lent me your servant on constraint, but willingly.

But without thy mind would I do nothing,.... Which shows great modesty and humility in the apostle, that though as such he had an authority, which he could have used, as well as had understanding and judgment how to have used it without consulting Philemon, or having his sense of this affair, yet chose to consult him: and it also shows the strict regard the apostle had to equity and justice, that he would do nothing with another man's servant without his consent; he would not seem to alienate, or engross another man's right and property, whatever power he might have, as an apostle, to have retained Onesimus as a minister to him,
That thy benefit should not be as it were of necessity, but willingly; that is, that his goodness in forgiving his servant, and renouncing all claim and property in him, and admitting him to continue in the service of the apostle, might not look like a forced thing; but that it might appear to be a voluntary action, when he should of himself return him, after he had been thus sent to him, and received by him.

without thy mind--that is, consent.
should not be as--"should not appear as a matter of necessity, but of free will." Had Paul kept Onesimus, however willing to gratify Paul Philemon might be, he would have no opportunity given him of showing he was so, his leave not having been asked.

That thy benefit might not be by constraint - For Philemon could not have refused it.