Repairing an Argument

Example Argument: Jerry and Terry used to be the same weight. Jerry now weighs more than Terry. Therefore Jerry must have gained weight.

Analysis: It is not valid to conclude that Jerry gained weight especially since Terry could have just lost weight resulting in Jerry weighing more. In other words, a weight gain by Jerry or a weight loss by Terry, are both possible reasons why Jerry weighs more than Terry. Not knowing the exact weights of the two gentlemen makes it hard to distinguish what really happened. Therefore an additional premise is needed. Including "Terry stayed the same weight as before" would make this a good argument because it validates the conclusion. It also eliminates the possibility that Terry could have lost weight. In order for  the conclusion of Jerry gaining weight to be true, all of the premises, including the state of Terry's weight, must be true.