Seemingly invalid piece of reasoning in The Republic

Question

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)

Certainly.

And he who is most skillful in preventing or escaping from a disease is best able to create one? (2)

True.

And he is the best guard of a camp who is best able to steal a march upon the enemy? (3)

Certainly.

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?

Answer 1

In context, these aren't an example of a linear argument, but four separate a illustrations of the idea that one who is good at doing one part of an occupation well is also able to other parts well. In fact Socrates introduces the idea with this phrase:

Then, my friend, justice cannot be a thing of much worth if it is useful only for things out of use and useless. (333e)

It seems Plato (via Socrates) did mean these examples to be persuasive, but they are a part of a larger reductio ad absurdum that ends:

"So justice, according to you and Homer and Simonides, seems to be a kind of stealing, with the qualification that it is for the benefit of friends and the harm of enemies. Isn't that what you meant?"

"No, by Zeus," he replied. "I no longer know what I did mean. Yet this I still believe, that justice benefits friends and harms enemies." (334b)

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I'm not sure what translation you are using, but the question you marked at (3) doesn't seem properly translated. Paul Shorey translated the question as:

Is it not also true that he who best knows how to guard against disease is also most cunning to communicate it and escape detection? (333e)

The idea seems to be that one who knows how a disease is spread will have the knowledge of how to avoid spreading it as well as the knowledge of how to secretly infect people. I don't think there is any question but that this is true.