Mark and Theo Make Their Case is the third book in the Royal Fireworks philosophy curriculum. Readers will remember Mark and Theo from the first two books, and they will delight to find them brought together as they debate against Will, the architect of their world who is also the star of Will Power, a book further on in the series. Will has decided that the world he drew for Mark and Theo is imperfect and therefore is not worth maintaining. Mark and Theo argue against this idea, insisting that something that is not perfect can still be good. Children learn about principles originally put forth by Plato in his quest for a perfect world and Aristotle's rejection of that world. They learn how to identify a problem, make an argument, and make objections. In the process, they watch Mark and Theo work together toward a surprising conclusion that ultimately convinces Will that, though imperfect, they are perfect as they are.