In the beginning of The Republic, when discussing the nature of justice, Socrates leads Polemarchus to agree with him in this line of reasoning (the numbering is mine):
But let us consider this further point: Is not he who can best strike a blow in a boxing match or in any kind of fighting best able to ward off a blow? (1)
And he who is most skillful in preventing or escaping from a disease is best able to create one? (2)
And he is the best guard of a camp who is best able to steal a march upon the enemy? (3)
Then he who is a good keeper of anything is also a good thief? (4)
I really do not see how can you infer (2), (3) and (4) from (1). In fact, I'm not quite convinced that (1) is true, either. Is this a valid piece of reasoning, or is Plato purposefully presenting a non-sequitur?