On the contrary, The Philosopher (Ethic. v, 2) distinguishes theft from robbery, and states that theft is done in secret, but that robbery is done openly.
I answer that, Theft and robbery are vices contrary to justice, in as much as one man does another an injustice. Now "no man suffers an injustice willingly," as stated in Ethic. v, 9. Wherefore theft and robbery derive their sinful nature, through the taking being involuntary on the part of the person from whom something is taken. Now the involuntary is twofold, namely, through violence and through ignorance, as stated in Ethic. iii, 1. Therefore the sinful aspect of robbery differs from that of theft: and consequently they differ specifically.
Reply Obj. 1: In the other kinds of sin the sinful nature is not derived from something involuntary, as in the sins opposed to justice: and so where there is a different kind of involuntary, there is a different species of sin.
Reply Obj. 2: The remote end of robbery and theft is the same. But this is not enough for identity of species, because there is a difference of proximate ends, since the robber wishes to take a thing by his own power, but the thief, by cunning.
Reply Obj. 3: The robbery of a woman cannot be secret on the part of the woman who is taken: wherefore even if it be secret as regards the others from whom she is taken, the nature of robbery remains on the part of the woman to whom violence is done. _______________________
FIFTH ARTICLE [II-II, Q. 66, Art. 5]
Whether Theft Is Always a Sin?
Objection 1: It would seem that theft is not always a sin. For no sin is commanded by God, since it is written (Ecclus. 15:21): "He hath commanded no man to do wickedly." Yet we find that God commanded theft, for it is written (Ex. 12:35, 36): "And the children of Israel did as the Lord had commanded Moses [Vulg.: 'as Moses had commanded']. . . and they stripped the Egyptians." Therefore theft is not always a sin.
Obj. 2: Further, if a man finds a thing that is not his and takes it, he seems to commit a theft, for he takes another's property. Yet this seems lawful according to natural equity, as the jurists hold. [*See loc. cit. in Reply.] Therefore it seems that theft is not always a sin.
Obj. 3: Further, he that takes what is his own does not seem to sin, because he does not act against justice, since he does not destroy its equality. Yet a man commits a theft even if he secretly take his own property that is detained by or in the safe-keeping of another. Therefore it seems that theft is not always a sin.