The problem of other minds has traditionally been regarded as an epistemological challenge raised by the skeptic. The challenge may be expressed as follows: given that I can only observe the behavior of others, how can I know that others have minds? The thought behind the question is that no matter how sophisticated someone's behavior is, behavior on its own is not sufficient to guarantee the presence of mentality. It remains possible, for example, that other people are actually nothing more than automata made out of flesh (or "philosophical zombies" as the term for this example stands). Perhaps the main argument offered against this possibility in the history of philosophy is the argument from analogy (other things have minds if they are sufficiently similar to us); it can be found in the works of John Stuart Mill, A. J. Ayer, and Bertrand Russell. The argument from analogy has faced scrutiny from the likes of Norman Malcolm who have issues with the "one case" nature of the argument.