A document I saw on the web last week illustrates a thousand others like it. It pretends to celebrate the life and thought of Thomas Jefferson, and ends the litanies with several quotes from him. Except for one of these I knew and some that could be genuine, the rest were suspicious. I tested two, chosen more or less at random. One was a paraphrase, the other one was fake.

The underlying problem of this sort of enterprise seems to lie in an invalid syllogism: Statement P is true; Jefferson only made true statements; therefore, Jefferson could have made statement P. Granting the premises, the conclusion is valid but useless. The corollary “he said it” is clearly invalid.

This approach can be seen as a simple case of circular reasoning. Consider the claim “Statement P is true, therefore Jefferson could (or must) have said it.” But how does the speaker know that statement P is true? I suspect he would say that it is “because Jefferson said it.” In short, the quote comes from Jefferson because it is true, and it is true because it comes from Jefferson.

It seems that many people’s whole political-philosophical outlook is based on faulty logic, circular reasoning, and false information. Believing them on their word has, for some others, the benefit of confirming their prior biases. By giving each person his own printing press, social media has deepened the problem.

It is often historically obvious that a quote cannot be genuine—when, for example, Cicero is quoted about the government deficit, which could not exist at the time of the Roman Republic. In other cases, following a simple principle prevents the danger: if a quote has no credible source, assume it is spurious, intentionally or not. (Some sources are more credible than others, whether or not they echo what you believe.) A person who wants you to believe a statement and who has some respect for you will make sure that the statement meets some minimum standards of truth.

With AI’s capacity to manufacture “deep fakes”—instead of the ordinary fakes I have been speaking of—much worse is coming. Can we hope that people will have the ability to be critical of the information they receive without being cynical (“Everything is false, except what I know in my guts”)? This is not merely a psychological danger. From those who think that they have a right to prevent other individuals from living peacefully as they want to, it is a mortal political risk.