A priori vs a posteriori

 

There are a number of ways to understand this distinction, but central to any discussion is the issue of justification. How do we defend the claims we make? To what must we appeal in order to establish their plausibility, to persuade others (in a rational way) to believe them?

A priori: a claim is justified a priori if the reasons for thinking it true are independent of experience.

A posteriori: a claim is justified a posteriori if the reasons for thinking it true are dependent on experience.

Very roughly, the idea is that a posteriori claims require that we check the world to establish their truth, but that a priori claims do not require this. If establishing a claim requires empirical research, it is a posteriori. If it doesn't, it's a priori.

A priori or a posteriori?

  1. All sisters are female.
  2. Smoking causes lung disease in humans.
  3. If Su-Feh is taller than David, and David is taller than Nandini, then Su-Feh is taller than Nandini.
  4. 41 is a prime number.
  5. My dog has big feet.
  6. Most redheads have bad tempers.
  7. Consciousness is epiphenomenal.

(see below for "answers")

There is a sense in which the difference is relative to epistemic position. If I already know (have good reason to believe) that all horses are mammals and that Northern Dancer is a horse, then I am justified in thinking that Northern Dancer is a mammal, without having to do further empirical research. Of course, ultimately, my belief is justified only to the extent that my beliefs in the "premises" are justified.

Science and Philosophy: At one point in our intellectual history, philosophy was considered to encompass all of the study of the nature of reality (and, perhaps, more). Empirical research was "natural philosophy". How can modern philosophy be of help with empirical questions? What can one learn without laboratories, research assistants, and generous funding? What rôle do philosophers play?

¨ "It is clear that in scientific society, the philosophers function as a sort of judiciary, an intellectual police force that remind us that things may not always be as simple as they [the things, not the philosophers] seem." (Hobson, 1999, p. 108)

¨ "Thought experiments" can inform us of the way the world must, could, or is likely to, be. "These arguments from thought experiments are only plausibility arguments, as always, but I think they have considerable force." (Chalmers, 1996, p. 250). Some think that such arguments can establish claims a priori. "Galileo showed that all bodies fall at the same speed with a brilliant thought experiment that started by destroying the then reigning Aristotelian account. The latter holds that heavy bodies fall faster than light ones (H > L). But consider [this figure], in which a heavy cannon ball (H) and light musket ball (L) are attached together to form a compound object (H+L); the latter must fall faster than the cannon ball alone. Yet the compound object must also fall slower, since the light part will act as a drag on the heavy part. Now we have a contradiction. (H+L > H and H > H+L) That's the end of Aristotle's theory; but there is a bonus, since the right account is now obvious: they all fall at the same speed (H = L = H+L)." (Brown, 1991, from his homepage)

Without exhausting options for the rôle of philosophy, let alone arguing for one in particular, it seems fair to say that science requires both a posteriori and a priori claims. For example, an answer to the question of how a given sort of claim should be justified will itself involve a priori considerations.

Answers?

  1. A priori
  2. A posteriori
  3. A priori
  4. A priori
  5. A posteriori
  6. A posteriori
  7. ????

On to next page, the Argument from Illusion.

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