Philosophy 1110 (Introduction to Philosophy) Dr. Yalçin

Empirical equivalence: Two (or more) beliefs (hypotheses, theories) are empirically equivalent if and only if whatever justifies one to some degree also justifies the other to the same degree. (in other words, one of them can never be better justified than the other)

Underdetermination: Two (or more) beliefs (hypotheses, theories) are underdetermined if and only if they are empirically equivalent and they cannot be both true (i.e., they are incompatible).

Phyrro's Principle: If two (or more) beliefs (hypotheses, theories) are justifed to the same degree, neither is adequately justifed (for knowledge).

Example: Think of one of the familiar ways a defense attorney can defend her client, say John, in court. Let us say that the client is charged with murder, and that the prosecutor provides the jury with a bunch of evidence. Now, the prosecutor claims that the evidence justifies the hypothesis that the John is the killer. But, if the defense attorney can demonstrate that the same evidence justifies the hypothesis that someone else, say Harry, is the killer to the same degree, this is considered to be an adequate defense. Here is how the defense attorney's argument will go:

(1) The two hypotheses (H1= John is the killer; H2= Harry is the killer) are underdetermined.
(2) If If H1 and H2 are justifed to the same degree, then neither is adequately justifed. (from Phyrro's Principle)
\ (3) Neither H1 nor H2 are adequately justified.
(4) Knowledge requires an adequate level of justification (from the definition of knowledge).
\ (5) We (including the jury) do not know that H1 is true (on the basis of the evidence available to us).

The point to remember is that the skeptical argument proceeds exactly in the same manner!

H1 = I am having these experiences because there are tables and chairs (and no Evil Demon) that are causing me to undergo these experiences.
H2 = I am having these experiences because there is a very powerful Evil Demon ( and no tables and chairs) that is causing me to undergo these experiences.
The skpetic claims that H1 and H2 are empirically equivalent given the kind of evidence a person can have. Since, obviously, they cannot be both true, we get premise (1), and the rest of the argument:

(1) The two hypotheses H1 and H2 are underdetermined (given the kind of evidence a person can have.
(2) If H1 and H2 are justifed to the same degree, then neither is adequately justifed. (from Phyrro's Principle)
\ (3) Neither H1 nor H2 are adequately justified.
(4) Knowledge requires an adequate level of justification (from the definition of knowledge).
\ (5) We do not know that H1 is true (on the basis of the evidence available to us).
\ (6) We do not know that there are tables and chairs (in other words, we do not know that the world outside of our own mind is the way we believe it to be).

The externalist answer to skepticism: The key idea of the externalist is that your beliefs can be justified even if you have no idea that they are justified, or how they are justified. A good example of an externalist is the reliabilist, who gives the following definition of justification:

John is justified in his belief that p (say, that the roses have bloomed) if and only if: John's belief (that the roses have bloomed) is produced by (caused by, generated by, is a result of) the operation of a highly reliable belief producing mechanism in John.

Most reliabilists would say that a belief producing mechanism is highly reliable if the mechanism produces true beliefs a high percentage of the time. Examples of belief producing mechanisms that are candidates for being highly reliable are: perceptual mechanims (seeing, hearing, etc.)--examples of belief producing mechanisms that are not very good candidates for being highly reliable are: long-term memory, gut-feelings, consulting the astrology column of a newspaper.

At this point, the externalist/reliabilist has the following answer to skepticism: Assume that the world is as H1 says it is (there are tables, chairs, etc.) This would make H1 a true belief. In addition, if H1 is also produced in those people who believe it by a relaible belief producing mechanism (and remember, the externalist does not require you to know that it is produced that way), H1 will also be justified. Being a justified true belief, it will constitute knowledge.

One last example: (Assume that we become so good in physiology that we can do this) We implant a gadget into a person's brain (she does not know that we have done this). This gadget periodically (a) measures the ambient temperature, C, and (b) makes the person believe that the temperature is C degrees. Now, let us say that as a result of the operation of this highly reliable gadget (which is a highly reliable belief-forming mechanism), our friend one day finds herself believeing that it is 72 degrees out there (and her belief is true). The reliabilist would say that the belief is also justified, and hence that she knows that it is 72 degrees out there. (despite the fact that she has no idea why she belives this, doesn't suspect the existence of the gadget and so on).