About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Monday, July 02, 2012

RS encore: Carneades, the quasi-lost skeptic

by Massimo Pigliucci

Few non-philosophers have ever heard of Carneades (214-129 BCE), and yet in some sense he was the founder of the type of thinking that eventually led to modern science. To understand why Carneades is so relevant, we need to briefly examine the context of his contribution to epistemology (i.e., his theory of knowledge). Carneades was the head of the Academy, the school founded by Plato, and Academics had long been embroiled in a debate with the Stoics that went something like this. The Stoics maintained that what they called “cognitive impressions” (i.e., sense data about the world) are, under ideal circumstances, a firm foundation for knowledge. In a sense, this anticipated the empiricist position of the 18th century according to which all knowledge is, ultimately, derived from our senses.

But the Academics replied that we know of many cognitive impressions that are false or misleading, for example dreams, optical illusions and (interestingly) visions of the gods. Classical Academics therefore arrived at the radical skeptical conclusion that one cannot really know anything for sure and that, consequently, the wise person ought to abstain from formulating any opinion.

Ah, the Stoics replied, but if you do that all human activities and forms of inquiry are impossible, clearly an unsustainable state of affairs, especially for philosophers. Moreover – with an argument anticipating modern criticism of extreme deconstructionism – they pointed out that the Academics' stance was self-contradictory: if we can't know anything for sure, how can we conclude (i.e., know) that the best position is to abstain from opinion?

To most non-philosophers this must sound like a classic example of “academic” (in the pejorative sense of useless) debate, but indulge me for a couple of more minutes. Carneades' most important contribution to epistemology was to find a way to overcome the impasse. His solution strikes us moderns as eminently sensible and even obvious, but so does Copernicus' insight that the Sun, not the Earth, is at the center of the solar system.

Carneades reckoned that we don't actually need certain knowledge in order to function and conduct inquiry, all we need is an estimate of how probable a given conclusion is. Indeed, the word “probability” comes from the Latin probabilis, Cicero's translation of the Greek word used by Carneades, “pithanos” -- which means persuasive. That is, in order to form reasonable opinions, we need persuasive evidence. Carneades suggested that a given conclusion can be more or less persuasive (probable) depending on two factors: how well if fits with other components of our knowledge of the world, and how much time and resources we are willing to devote to further consolidate the conclusion itself by additional inquiries. The more we wish to be confident of our assessment (because it is more important for us for whatever reason), the more effort we need to be willing to invest in the pursuit. The reasonable skeptic, then, is justified in holding opinions about things in direct proportion with the persuasiveness of the evidence, as well as in being more demanding of evidence in proportion to the importance of the matter at hand.

As I mentioned earlier, this insights provides the foundations of modern scientific reasoning, and it is one of the most difficult things to communicate to the general public. Take, just because this is Darwin's week, the case of creationism. What many creationists honestly do not appreciate is that they are not just attacking a specific scientific theory: because of the many interconnections among modern sciences, they are rejecting a whole edifice to which a variety of disciplines have contributed over the past few centuries, from physics to astronomy, from chemistry to geology. It isn't that it is impossible for evolutionary theory to be wrong, and it surely isn't that scientists are certain of its truth. It is, as Carneades would have said, that the evidence in favor of the theory is so massive that any reasonable person ought to provisionally assent to its truth.


  1. This may work well enough for phenomena that can be independently observed. Sensory data is persuasive, so to dissuade us from believing it requires something extra, some other explanation that is positivist in nature. For example, our senses tell us that the moon is bigger when viewed just above the horizon than it is when higher in the sky. Hopefully, all of us would agree that the size of the moon doesn't actually change in such a fashion, and many of us could formulate extremely sound arguments to support the rejection of what our senses tell us. Some of those arguments would be far abstracted from any sensory data upon which they are built, and yet they would override the immediate impressions our senses provide. Indeed, for most people even wrong arguments (atmospheric lensing for example) would be more persuasive than what our senses tell us.

    But when it comes to the human world, or more precisely, the world of humans thinking about humans, we need something more. Why, for example, "ought" any reasonable person provisionally assent to the truth of a theory which is supported by a massive body of persuasive evidence, when none of it can be proven absolutely? One gets the sense that the Academics would never accept such a view. And yet, Carneades' approach is so intuitively compelling that their objections are immediately marginalized.

    I note that the persuasiveness of Carneades' approach suggests a commonality of mental architecture among humans thinking about humans - that is, self-reflective individuals who identify as members of a group called humans - that, while not universal, is nevertheless quite broad. Perhaps this commonality, which cannot be directly observed but facts about which can nevertheless be triangulated, is both extensive and stable enough for us to map it. Perhaps on this basis we may establish what we "ought" to accept as a basis for the attribute of "oughtness".

  2. Carneades' belief that our common experience of reality as mediated by the senses does not provide knowledge leaves open the possibility of knowledge attained by pure reason. The persuasion by the senses first depends upon "how well it fits with other components of our knowledge of the world..." which seems to mean logical/mathematical analysis of speculative metaphysics. That does not seem to be part of the modern scientific outlook, or at least, I doubt it should be.

    Further, the notion that a large amount of unreliable evidence (is that an oxymoron?) is somehow persuasive seems to rest upon the tacit assumption that reality is somehow coherent/intelligible/lawful. It implies therefore lots of evidence reveals this underlying pattern. It is entirely unclear to me upon what grounds anyone who rejects empiricism (in some form) rests this assumption.

    Carneades' second factor, "how much time and resources we are willing to devote to further consolidate the conclusion itself by additional inquiries..." either somehow concludes that accumulation of more unreliable evidence somehow answers the question of why we should think an apparent consistency is more than appearance.

    Or, possibly, that other types of knowledge than empircal/experiential evidence can be used to validate it, not perfectly enough to call it true, genuine knowledge but persuasive supposition. Rejecting empirical/experiential evidence as the touchstone of course means rejecting experimental evidence as well. Again, I'm not so sure that this is what scientists do, or should, believe.

    I don't think Carneades can be held as really accepting any kind of scientific outlook. His solution is to a problem that is founded on repudiation of any kind of scientific outlook in my opinion. The starting assumption that there is no way to distinguish valid sensory data from dreams and visions (also known as hallucinations) start from refusing to accept that you can verify the data from dreams and visions by asking other people what they see. Science does not proceed by repudiating the concept of objective (or should I say, intersubjective?) fact.

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  4. wonder if you have twisted Carneades slightly by saying "important" rather than "confident". Importance would imply value, which might be assumed also of something about which we want to be confident, but not necessarily for the cold professional. Nevertheless, evolution is something important demanding confidence, and I would agree also that it is about uncertainty, or probability, whether anything is true.


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