John Lightfoot on Matthew 12:2

[They do that which is not lawful to do on the sabbath day.] They do not contend about the thing itself, because it was lawful, Deuteronomy 23:25; but about the thing done on the sabbath. Concerning which the Fathers of the Traditions write thus; "He that reaps on the sabbath, though never so little, is guilty. And to pluck the ears of corn is a kind of reaping; and whosoever plucks any thing from the springing of his own fruit is guilty, under the name of a reaper." But under what guilt were they held? He had said this before, at the beginning of chapter 7, in these words: "The works whereby a man is guilty of stoning and cutting off, if he do them presumptuously; but if ignorantly, he is bound to bring a sacrifice for sin, are either primitive or derivative" Of 'primitive,' or of the general kinds of works, are nine-and-thirty reckoned; "To plough, to sow, to reap, to gather the sheaves, to thrash, to sift, to grind, to bake, &c.; to shear sheep, to dye wool," &c. The derivative works, or the particulars of those generals, are such as are of the same rank and likeness with them. For example, digging is of the same kind with ploughing; chopping of herbs is of the same rank with grinding; and plucking the ears of corn is of the same nature with reaping. Our Saviour, therefore, pleaded the cause of the disciples so much the more eagerly, because now their lives were in danger; for the canons of the scribes adjudged them to stoning for what they had done, if so be it could be proved that they had done it presumptuously. From hence, therefore, he begins their defence, that this was done by the disciples out of necessity, hunger compelling them, not out of any contempt of the laws.