A dialogue between four people who represent different philosophical viewpoints: Sarah, the scientific fallibilist; Bob, the naïf, who values direct personal experience; Zac, the relativist; and Roxana, the logician.

The four meet on a train. Sarah finds Bob, with whom she is on friendly terms, with a cast on his leg. He explains that his garden wall collapsed on his leg, but when Sarah commiserates about his bad luck, he insists that it was not luck: his neighbor used some mystical means to cause the wall to collapse. Sarah protests that there is surely a more logical, scientific explanation for the wall's collapse, but Bob is adamant:

The two argue about it for a while, and Zac overhears and steps in, asking them to consider the relativist position, that both of them are correct, from their own points of view. Sarah protests that they cannot both be correct, when they are advancing incompatible ideas. Zac responds:

Zac takes issue with absolutist notions of 'true' and 'false'. Roxana arrives and, hearing the argument, comments that they do not seem to understand logic, and offers a definition from Aristotle:

There is no point, she says, making an assertion but refusing to say that one's assertion is true.

During the balance of the book, they discuss epistemology and then ethics–primarily the question of whether there is some basis for ethics, and whether absolutist or relativist ethics is valid. Sarah initially takes a relativist position on ethics, but she is unwilling to take an absolutely relativist position on morals (as one might with regard to etiquette). For example, she says that she would support intervention to free slaves, even if the slaveholders believe they are behaving morally and cannot be convinced otherwise through discussion:

Generally, it is Sarah's views being tested: the other three are primarily there to object to weaknesses in her arguments. It's a bit of a shame, really, that the others don't get to advance meaningful arguments except relative to Sarah's viewpoint.

Roxana (who is something of a caricature of a logician) objects when the others make some illogical argument, but offering very little herself. She might profitably have discussed formal reasoning, but she leaves it at "I listen to their dispute merely as an instructive example of the chaotic nature of debate between untrained persons."

Zac often refuses to take any position, backing away with "it's my point of view" when pressed. The others call him on it, but he still doesn't offer anything but vague claims that relativism is going to have a huge impact on society, some day, and absolutism is what has led to wars and other abuses. His relativist position is certainly not presented as terribly useful or defensible.

Bob has the most obviously wrong position, from the start–that witchcraft is real–but he actually makes some very worthwhile arguments about gaining knowledge from direct experience, which the dialogue pursues. Do dogs have knowledge, gained from their senses? Is there a higher standard for human knowledge? Does knowledge require verbalization? To what extent must we accept knowledge that others claim they have gained through experience–for example, Bob's 'sensing' that a woman is a witch?

The topics covered in the dialogue are fairly basic, and are unlikely to be new to anyone who has studied epistemology and ethics even very casually, but they are presented in a way that makes the problems of philosophy very easy to understand–even if there are not any satisfying answers to those problems.

It was occasionally frustrating when a character would make a weak argument, clearly for the didactic purpose of exposing the weakness. If you couldn't see the weakness immediately, or anticipate that they were likely to make the bad argument, it wouldn't be so bad. This would probably not be an issue for someone new to the ideas.

Tetralogue should provide a good introduction for a beginner wondering what this philosophy stuff is all about.