Clearly.

And when you want to keep a shield or a lyre, and not to use them, you would say that justice is useful; but when you want to use them, then the art of the soldier or of the musician?

Certainly.

And so of all other things; — justice is useful when they are useless, and useless when they are useful?

That is the inference.

Then justice is not good for much. 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?

Certainly.

And he who is most skilful in preventing or escaping from a disease is best able to create one?