You shall not steal.
For some strange reason, we find it easier to steal if our victims have no faces. One $20 bill stolen from a stranger on the bus next to you is theft, but money stolen by downloading music off the Internet that you haven’t paid for is free for the taking. Indeed, for some people, “sharing” music without paying for it has somehow become a positive “right.” And the justification for that right is that the people we are stealing from have “enough money already.” Now, if it were a matter of a poor man with no hope of gain putting bread in his baby’s mouth by stealing from a miser who has more than he needs, you might have a case in Catholic understanding. As St. John Chrysostom says, “Not to enable the poor to share in our goods is to steal from them and deprive them of life. The goods we possess are not ours, but theirs.” So there is room for Robin Hood in the Catholic tradition. But your average MP3 collector is not stealing because of some noble principle of helping the poor. He’s just taking stuff he wants because he’s too much of a slob to pay for it. There is no difference between that and stuffing CDs into our pocket at the local music store, except for one thing: music store owners have faces and we can see our theft reflected in their outraged expressions clearly.