Aristotle, after an exhausting list of topics and their use in argumentation, offers us a list of fallacies in enthymematic reasoning. If all enthymematic reasoning is based on probabilities and assumptions that the audience is willing to grant, we must wonder, Are there really “fallacies” arguments in rhetoric as there are in dialectic? In the 20th century, Chaim Perelman argued that there is really only one rhetorical fallacy (petitio principii): the assumption of premises that the audience is unwilling to grant. Take the post hoc ergo propter hoc fallacy, as Aristotle discusses it on p. 234. If we say that something caused something else because it followed after, Aristotle would say, we are committing a fallacy. But we might be willing to accept that certain kinds of things typically follow from other kinds of things. So, if the audience is willing to accept that the quality of healthcare in a given country can affect the population’s mortality, then we may be likely to accept that a higher mortality following a change in healthcare policy is a result of that policy. It seems reasonable to change the policy, even though we may not have any direct evidence of a causal connection. Have we committed a fallacy in the absolute sense of the term?

Put the question another way. Is Aristotle right—are there hard and fast fallacies that can be identified and debunked? If so, what are they, and what good is it to the judge or the advocate to learn them? Or, is Perelman right, are there no hard and fast fallacies, only mistaken assumptions about what the audience already believes? If so, how can we judge an argument right or wrong? Are we left with the simple assertion of “effective” or “ineffective” depending on the audience’s assumptions?