Can a bare thought be evil? Charnock in his essay The Sinfulness and Cure of Thoughts makes this observation:
First motions: those unfledged thoughts and single threads, before a multitude of them come to be twisted and woven into a discourse; such as skip up from our natural corruptions, and sink down again, as fish in a river. These are sins, though we consent not to them, because, though they are without our will, they are not against our nature, but spring from an inordinate frame, of a different hue from what God implanted in us. How can the first sprouts be good, if the root be evil? Not only the thought formed, but the very formation, or first imagination, is evil. Voluntariness is not necessary to the essence of a sin, though it be to the aggravation of it. It is not my will or knowledge which doth make an act sinful, but God’s prohibition. Lot’s incest was not ushered by any deliberate consent of his will, Gen. 19: 33, 35, yet who will deny it to be a sin, since he should have exercised a severer command over himself than to be overtaken with drunkenness, which was the occasion of it? Original sin is not effectivè voluntary, in infants, because no act of the will is exerted in an infant about it; yet it is voluntary subjectivè, because it doth inhærere voluntati. These motions may be said to be voluntary negatively, because the will doth not set bounds to them, and exercise that sovereign dominion over the operations of the soul which it ought to do, and wherewith it was at its first creation invested. Besides, though the will doth not immediately consent to them, yet it consents to the occasions which administer such motions, and therefore, according to the rule, that causa causæ est causa causati, they may be justly charged upon our score.
They [sinful thoughts] are contrary to the law, which doth forbid the first foamings and belchings of the heart, because they arise from an habitual corruption, and testify a defect of something which the law requires to be in us, to correct the excursions of our minds: Rom. 7: 7, ‘I had not known lust, except the law had said, Thou shalt not covet.’ Doth not the law oblige man as a rational creature? Shall it then leave that part, which doth constitute him rational, to fleeting and giddy fancies? No; it binds the soul as the principal agent, the body only as the instrument. For if it were given only for the sensitive part, without any respect to the rational, it would concern brutes as well as men, which are as capable of a rational command and a voluntary obedience, as man without the conduct of a rational soul. It exacts a conformity of the whole man to God, and prohibits a deformity, and therefore engageth chiefly the inward part, which is most the man. It must then extend to all the acts of the man, consequently to his thoughts, they being more the acts of the man than the motions of the body.
We are accountable to God, and punishable for thoughts. Nothing is the meritorious cause of God’s wrath but sin. The text tells us, that they were once the keys which opened the flood-gates of divine vengeance, and broached both the upper and nether cisterns, to overflow the world. If they need a pardon— Acts 8: 22, ‘If perhaps the thought of thy heart may be forgiven thee’—( as certainly they do), then, if mercy doth not pardon them, justice will condemn them. And it is absolutely said, Prov. 12: 2, ‘That a man of wicked devices,’ or thoughts, ‘God will condemn.’ It is God’s prerogative, often mentioned in Scripture, to ‘search the heart.’ To what purpose, if the acts of it did not fall under his censure, as well as his cognisance? He ‘weighs the spirits,’ Prov. 16: 2, in the balance of his sanctuary, and by the weights of his law, to sentence them, if they be found too light. The word doth discover and judge them: Heb. 4: 12, 13, ‘It divides asunder the soul and spirit,’ the sensitive part, the affections, and the rational, the understanding and will; both which it doth dissect, and open, and judge the acts of them, even the thoughts and intents, ἐνθυμήσεων καὶ ἐννοιῶν, whatsoever is within theθυμὸς, and whatsoever is within theνοῦς, the one referring to the soul, the other to the spirit.