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.

Friday, February 10, 2006

Carneades, the quasi-lost skeptic

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. Not having a formal education in philoshophy, I really enjoy posts like this.


  2. Would it be reading too much into his method to conclude that he was 'getting at' the Bayesian approach to probability?

  3. "The reasonable skeptic, then, is justified in holding opinions about things in direct proportion with the persuasiveness of the evidence"

    But induction was discredited by Hume over 200 years ago!

  4. Julius,

    Hume didn't discredit induction. Indeed, he said repeatedly that it is the only reasonable thing to do. What he did away with is the idea that there is any certain foundation for empirical knowledge.

  5. Did he not go further and establish that there is no foundation for empirical knowledge, whether certain or otherwise?

  6. Forgive my philisophical naivete, but isn't it possible Hume was just plain wrong. Granted, induction doesn't provide us with ABSOLUTE knowlege, but it is still a useful tool. Without it, pretty much all science is null and void.

    No more treatments when your sick, no more putting criminals in prison due to forensics, no more new technologies.

    Am I wrong?


  7. Looking at it from the other end, if we begin with absolute skepticism, what account would we give of the fact that we seem to know things, and that this knowledge seems to increase from time to time.

  8. Noah:

    Induction is logically incoherent. Science works perfectly well without it.


    Critical rationalists (aka "radical sceptics") do not deny the existence of truth. We are not relativists! However,we deny that it is possible to "justify", "confirm" or "verify" theories. Hence it is a waste of time trying. All our knowledge is conjectural, although if we lucky, what we conjecture is true may actually be true. What is important is therefore to eliminate false theories - by testing and criticism. And that is what scientists actually do, even if they don't know it!

  9. Noah,

    actually philosophers have done a pretty good job (beginning with Hume) at showing that science not only uses, but is ultimately founded, on induction. That's the so-called "problem of induction," which does make scientists rather uncomfortable. But, again, Hume (and all modern philosophers of science) still concludes that induction is the best we can do, we just need to learn to live with it!

  10. "actually philosophers have done a pretty good job (beginning with Hume) at showing that science not only uses, but is ultimately founded, on induction."

    Since induction is false (as Hume demonstrated), either science is founded on falsehood, or your statement that science is founded on induction is false.

    I suggest it is the latter rather than the former!


  11. Julius, Hume most certainly did NOT demonstrate that induction is false. He showed that it cannot lead to certain knowledge, and so neither can science (in fact, nothing can, except for logic and mathematics, which are independent of the empirical world). There is a HUGE difference.

  12. "It is only experience, which teaches us the nature and bounds of cause and effect, and enables us to infer the existence of one object from that of another."
    -Hume, Enquiry..., 132

  13. Julius,

    I haven't read Hume. I would like to, but I have very long list of books I plan to read and probobly won't get to him for another couple of years.

    Do you mind explaining how induction is "logically incoherent" or "false" for the philisophicaly ingnoant? There seems to be some disagreement on the subject and blanket statements don't really clarify the issue.


  14. Noah: inductive "logic" reasons from the particular to the general. The idea is that if some members of set X are observed to be white than the remaining members of set X are also white. It is logically invalid because the premise can be true even though the conclusion is false.

  15. Massimo, as you well know, Hume recognised that induction was false, but concluded that nevertheless we need it. Hence "the problem" of induction.
    Popper showed we don't need it; and hence there is no "problem" of induction. We can now safely ignore it both because it is false and because it is unnecessary.

  16. Julius,

    that's not my reading of the situation. Hume, again, showed that induction cannot guarantee truth. This is not at all the same as showing its falsity.

    Second, Popper thought he solved the problem of induction with his idea of falsification, but subsequent philosophers -- beginning with his student Lakatos -- have shown that falsification doesn't work, and hence doesn't solve the problem of induction.

    Falsification of a hypothesis is not possible because if the data don't fit the hypothesis it is always feasible to go back and "rescue" the hypothesis by modifying it or some of its corollary assumptions. Science proceeds instead by a mix of induction and deduction, which doesn't guarantee truth but only likelihoods.

  17. 1. I'll leave Hume to one side. Interesting though it may be, ultimately what he did or didn't say does not affect the truth or falsity of induction.

    2. I have not read Latakos so cannot comment; except that it is not my understanding that Lakatos falsified falsification (as it were)

    3. I don't understand why you say that this possibility renders falsification impossible or even problematic, as long as the modified hypothesis is still criticisable.


  18. Lakatos (and others) pointed out that if the crucial hypothesis is not directly falsifiable, then one has to fall back on a combination of supportive and unsupportive evidence, and balance them against other competing hypotheses -- exactly what Popper wanted to avoid.

  19. OK, here's what I understand;

    Deduction: if premises true then inference is true

    Induction: if premises true then inference is probably true

    In most of science (and life) we can never do better than “probably” true

    and the whole idea that the term/action of deduction or induction can be true or false makes no sense to me. Better to describe them as actions that "work" or "don't work." (which itself is too black & white, reality is more a sliding scale...)

  20. That's what I'm thinking "me".

    It seems to me that conclusions reached by induction are PROVISIONALY true.

    Julius' explaination of the falsity of induction seems to only apply SITUATIONALLY. Maybe I'm misunderstanding him.


  21. I always get confused by these phylosophical musings, since I have no competence on the subject. But I'll gladly give my opinion anyway... :-)

    Whatever science uses, it seems to be kinda working, I guess everyone would agree.

    Julius said:
    inductive "logic" reasons from the particular to the general. The idea is that if some members of set X are observed to be white than the remaining members of set X are also white.

    I will believe you when you say this is logically invalid. The problem is that this is what scientists do all the time. We go from the particular to the general. Then we hypothesize that members of X are white - which holds until we see non-white members. I don't know if this is good practice or not, but as far as my rudimentary judgement of the phylosophical foundations of what I do go, it seems to me that's what we've been doing all along.

    But I might be missing some subtlety there and we're talking different things...


  22. "Lakatos (and others) pointed out that if the crucial hypothesis is not directly falsifiable..."

    Falsifiability is not the only method of refutation. Criticism can take many forms; and it is hard to conceive of a scientific (in the wide sense) hypothesis that is immune to criticism.

  23. "I will believe you when you say this is logically invalid. The problem is that this is what scientists do all the time. We go from the particular to the general."

    You need to distinguish between two things:

    1. Coming up with ideas;

    2. Distinguishing the false ideas from the true ones.

    There is an infinite number of different ways of coming up with new ideas; and it may be that generalising from particular instances is a technique that humans use (although I doubt it is widespread - most new ideas are devised in order to explain anomolies).

    But it is the second stage, which is what this debate is about. The philosophical question is how to find out which of the ideas that people have conjectured are false and which are true. Induction is useless for this purpose because generalising from "I have seen four white swans" to "all swans are white" or even "all swans are probably white" is logically invalid.

    Therefore whatever scientists may think they are doing, they cannot actually use induction to distinguish truth from falsehood in science; and it would promote clarity of thought if that were to be more widely appreciated.

  24. "Whatever scientists may think they are doing, they cannot actually use induction to distinguish truth from falsehood in science."

    Interesting statement, since this is exactly what scientists do, and apparently it works pretty darn well...

  25. "Falsifiability is not the only method of refutation. Criticism can take many forms..."

    Julius, I can see how the critical approach fits well with certain kinds of endeavours, e.g., biblical criticism. Which came first, Matthew, Mark or Luke? Scholars make plausible cases for all three positions, and combinations thereof. Direct falsification is probably not possible. All you can do is weigh the pros and cons of each position, and see which thesis survives unscathed, or with the least damage.

    But in a lot of scientific endeavours, scientists are looking for new ways of measuring things, or are seeking to apply the techniques that are already at their service. So with the use of the molecular clock to estimate times of divergence, genome sequencing to indicate degrees of relationship, measuring intensity of various astronomical objects to get at the age of the universe, or to study the nature of the universe at a certain time, not to mention radiological dating to establish the age of all sorts of things. Scientists actually use these techniques, and when they run into difficulties, the typical response is to sample more widely- a clear sign of induction. Conversely, if, for example, the phylogenetic tree established by molecular studies agrees with the tree established by morphological studies, this convergence is considered strong evidence in favor of the underlying theory. The results may be fallible, but to call them invalid would be unjust. It would also be wrong to say that this is not what scientists are really doing, especially when they express their results in statistical form.

  26. Not to be a pest, but when you're retired you have lots of time to think.

    It seems to me that the expansion of the universe is an inductive generalisation, if there ever was one.

    Also it's a scientific "fact", if there ever was one. We say we know the universe is expanding.

    It is also provisional. In a different theoretical framework (e.g., multiverse) this "fact" might be seen as a local phenomenon (universe redefined).

  27. I would like to add that it is not just scientists that use induction, modern industry practically depends on it when it comes to getting products out the door.

    For those who have never worked in industry, I'll explain.

    Most (if not all) mass production plants have quality control departments. For most parts being mass produced, a sample is taken from each lot and checked for critical dimensions and structural properties. If the parts in these samples meet specifications, the entire lot is assumed fit for shipping. In some cases an entire lot may need to be checked; in some cases the assumption of quality is proven false (bad parts get through;) but in any case, the inductive approach saves time and money for most mass production industries. If we were to check every peice every time, it would slow production to a crall and both buisnesses and consumers would suffer.

    I would say that makes induction pretty useful, at least to industry.


  28. Why not cut thru the evolution debate and talk about the fact that the willful disregard of evidence in order to benefit whether by inspiration, power, control or money--- at other's expense (condemnation, rejection, destroying reputations, etc) is completely dishonest. Judging religion purely on logic is like judging Hitler solely on military might. Let's frame the argument as it should be--whether people present them honestly or not.

  29. RE: Hitler, logic, religion and dishonesty.

    It's difficult to apply logic to a whole lot of stuff people choose for themselves. The rules found in the christain religion, for instance, are not in and of themselves "dishonest". It is more likely the (mis)application of any set of principles that make those principles effective or ineffective.

    The only notable connection between Hitler and "the Church" is that Hitler had drafted a good deal of his political propaganda off of an old (and false) Catholic doctrine referred to as "replacement theology".

    Replacement theology states that "The Church" supposedly replaces Israel due to Israel's rejection of Christ. Anyone who reads the texts on these matters from beginning to end without reasons to distort the context, knows plainly that the RCC was and still is engaing in a complete misapplication of verses and the progression of events that were intended firstly for the benefit of Israel, and then the Gentiles. But a complete reading of the Bible shows that God has never and will never abandon Israel.

    On the establishment of facts, chronology clearly matters. But in the absence of having the (teachable)heart and mind to read well enough to place things in their proper context, this may tend to make some usually straightforward issues appear to some to be illogical.

    What is astounding (and I would tend to think illogical) is that some people have unwittingly taken their anti-Semitism straight from their rejection of Catholism, right into their supposedly new and more 'rational' secularism.

    truth must begin wih the individual.

  30. As I understand it, Carneades held a probabilist approach to knowledge. However, I must also add it appears as if he was favouring abductive reasoning the most.


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