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.