What would you do if you had a magic ring that made you invisible, and which guaranteed that, whatever you did, you’d go unnoticed? Perhaps you’d spend your time like an invisible superhero, striking from nowhere to trip up bag-snatchers, using your power to expose criminal conspiracies by companies to use child slaves to make their products, or to dump toxic waste in rivers? If you did do things like this, would it bother you that no-one ever gave you even the tiniest bit of credit, or thanks, or even showed any acknowledgement at all of the fact that it was you that had done all of that? Would that not bother you? Not even a little bit? On the other hand, with the power of invisibility and a guarantee that you would never get caught, you could take what you wanted from anyone, at any time, anywhere. And you wouldn’t have to fear punishment, or shame, or retribution. What do you think? Which would it be?

In the Republic, Plato uses this question, and others like it, to make us think about why exactly we should be just, and be good. His profound answers to this question, as well as his further claims about how to organize society in a way that promotes justice and goodness in the state, are at the foundation of the discipline of philosophy. We will think and argue with Plato on the way to considering our own answers to these questions.

What do you now know most certainly of all? Perhaps you take yourself to know that there is a computer screen in front of you because you can see one? Or, perhaps you can take yourself to know that a car alarm is going off outside because you can hear one? Most of the things we know with certainty appear to come to us through the senses; through sight, smell and touch. But does all of our knowledge about the world come to us through the senses? Suppose that there was a powerful evil demon who has brought it about that the experiences that you are having now are all radically misleading about the real world. There is no computer, no cup of coffee on the desk, and no walls that surround you, even though it appears that there are. If all of the evidence of the senses cannot be trusted, is there anything at all that you are able to know in these circumstances? If so, how?

In the Meditations on First Philosophy, Descartes uses an exercise of this kind to show that it is possible to arrive at truths about the world independent of the use of the senses, simply through reasoning and reflection. This is an idea that places Descartes squarely in the Platonic tradition. But Descartes also combines his Platonism with the worldview of the new physics. What reason reveals—according to Descartes—is that the world is very different from the way it appears, lacking colour, taste, smell and sound, and composed only of extended stuff. Is he right?