Reply to Objection 2: By those words Christ wished to show that man is made unclean as to his soul, by the use of any sort of foods considered not in their nature, but only in some signification. And that certain foods are in the Law called "unclean" is due to some signification; whence Augustine says (Contra Faust. vi): "If a question be raised about swine and lambs, both are clean by nature, since 'all God's creatures are good'; but by a certain signification lambs are clean and swine unclean."
Reply to Objection 3: The disciples also, when, being hungry, they plucked the ears of corn on the Sabbath, are to be excused from transgressing the Law, since they were pressed by hunger: just as David did not transgress the Law when, through being compelled by hunger, he ate the loaves which it was not lawful for him to eat.