On the contrary, Isidore says (Etym. x): "Fur (thief) is derived from furvus and so from fuscus (dark), because he takes advantage of the night."
I answer that, Three things combine together to constitute theft. The first belongs to theft as being contrary to justice, which gives to each one that which is his, so that it belongs to theft to take possession of what is another's. The second thing belongs to theft as distinct from those sins which are committed against the person, such as murder and adultery, and in this respect it belongs to theft to be about a thing possessed: for if a man takes what is another's not as a possession but as a part (for instance, if he amputates a limb), or as a person connected with him (for instance, if he carry off his daughter or his wife), it is not strictly speaking a case of theft. The third difference is that which completes the nature of theft, and consists in a thing being taken secretly: and in this respect it belongs properly to theft that it consists in "taking another's thing secretly."
Reply Obj. 1: Secrecy is sometimes a cause of sin, as when a man employs secrecy in order to commit a sin, for instance in fraud and guile. In this way it does not diminish sin, but constitutes a species of sin: and thus it is in theft. In another way secrecy is merely a circumstance of sin, and thus it diminishes sin, both because it is a sign of shame, and because it removes scandal.
Reply Obj. 2: To keep back what is due to another, inflicts the same kind of injury as taking a thing unjustly: wherefore an unjust detention is included in an unjust taking.
Reply Obj. 3: Nothing prevents that which belongs to one person simply, from belonging to another in some respect: thus a deposit belongs simply to the depositor, but with regard to its custody it is the depositary's, and the thing stolen is the thief's, not simply, but as regards its custody. _______________________
FOURTH ARTICLE [II-II, Q. 66, Art. 4]
Whether Theft and Robbery Are Sins of Different Species?
Objection 1: It would seem that theft and robbery are not sins of different species. For theft and robbery differ as "secret" and "manifest": because theft is taking something secretly, while robbery is to take something violently and openly. Now in the other kinds of sins, the secret and the manifest do not differ specifically. Therefore theft and robbery are not different species of sin.
Obj. 2: Further, moral actions take their species from the end, as stated above (I-II, Q. 1, A. 3; Q. 18, A. 6). Now theft and robbery are directed to the same end, viz. the possession of another's property. Therefore they do not differ specifically.
Obj. 3: Further, just as a thing is taken by force for the sake of possession, so is a woman taken by force for pleasure: wherefore Isidore says (Etym. x) that "he who commits a rape is called a corrupter, and the victim of the rape is said to be corrupted." Now it is a case of rape whether the woman be carried off publicly or secretly. Therefore the thing appropriated is said to be taken by force, whether it be done secretly or publicly. Therefore theft and robbery do not differ.