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Justified Deduction Principle

• Remember: a valid, deductive inference guarantees the

truth of the conclusion given the truth of the premises.

• The justified deduction principle states that deductive

inference preserves not only truth but also justification.

Put another way, the justification of the premises and the

validity of the inference suffices to guarantee the

justification of the conclusion.

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Gettier‘s First Counterexample

• Suppose that Smith and Jones have applied for a certain job.

And suppose that Smith has strong evidence for the following

conjunctive proposition:

(d) Jones is the man who will get the job, and Jones

has ten coins in his pocket.

• Smith's evidence for (d) might be that the president of the

company assured him that Jones would in the end be

selected, and that he, Smith, had counted the coins in Jones's

pocket ten minutes ago. Proposition (d) entails:

(e) The man who will get the job has ten coins in his

pocket.

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• Let us suppose that Smith sees the entailment from (d) to

(e), and accepts (e) on the grounds of (d), for which he has

strong evidence. In this case, Smith is clearly justified in

believing that (e) is true.

• But imagine, further, that unknown to Smith, he himself, not

Jones, will get the job. And, also, unknown to Smith, he

himself has ten coins in his pocket. Proposition (e) is then

true, though proposition (d), from which Smith inferred (e), is

false.

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• In the example, then, all of the following are true:

(i) (e) is true

(ii) Smith believes that (e) is true

(iii) Smith is justified in believing that (e) is true.

• But it is equally clear that Smith does not know that (e) is

true; for (e) is true in virtue of the number of coins in

Smith's pocket, while Smith does not know how many

coins are in Smith's pocket, and bases his belief in (e) on

a count of the coins in Jones's pocket, whom he falsely

believes to be the man who will get the job.

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A Medieval Gettier Case

Peter Alboini of Mantua (d. 1400), De Scire et Dubitare in his Logica,

Venetiis 1492: Simon Bevilaqua

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“Let it be posited that Plato is very near you and you know that he is

running, but you believe that he is Socrates so that you firmly believe

that Socrates is running. But let Socrates in fact be running in

Rome, although you do not know this. You thus know that Socrates is

running and do not know that Socrates is running, therefore, on the

same basis, what is known is doubtful to you” (Logica, i7rB).

Helpful vs. Dangerous Gettier

Cases

• Helpful Gettier cases are ones where the lucky

occurrence functions beneficially in the sense that if the

lucky occurrence were absent, then (all else being equal)

the subject would not have a justified true belief. He

would lack the truth or the belief or the justification.

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Fake Barns. Henry is driving in a part of the country where,

unbeknownst to him, the inhabitants have erected a large

number of fake barns, i.e., paper-mâché facades looking like

barns from the highway, yet lacking back walls or interiors.

From the highway, these fake barns are indistinguishable

from real ones. Looking at what is in fact a real barn Henry

forms the belief that that is a barn. Does Henry know that

that is a barn?

• In dangerous Gettier cases (like Fake Barns) the lucky

occurrence functions as an unseen threat to the subject’s

having a justified true belief. If the lucky occurrence were

absent, then (all else being equal) the subject would not be

in any real danger of not having his justified true belief;

instead we would have a normal case of knowledge.

• If there were no fake barns in the vicinity, then (all else

being equal) Henry would truly and justifiably believe that

what he is looking at is a barn. So with the absence of the

strange occurrence (viz., the fake barns) and with all things

being equal, Henry would have a belief that is true and

justified in the normal way.

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Varieties of Epistemic Luck

Benign Cases of Luck:

• Content Epistemic Luck: It is lucky that the proposition is true.

• Capacity Epistemic Luck: It is lucky that the agent is capable of

knowledge.

• Evidential Epistemic Luck: It is lucky that the agent acquires the

evidence that he has in favor of his belief.

Malicious Cases of Luck:

• Veritic Epistemic Luck: It is a matter of luck that the agent’s belief is true.

• Reflective Epistemic Luck: Given only what the agent is able to know by

reflection alone, it is a matter of luck that his belief is true.

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Why are Gettierized Beliefs not

Knowledge?

• Smith holds a true belief but he could have very easily ended

up with a false belief. His belief is only accidentally true; it‘s

truth is not stable. Already slight changes in the situation make

Smith hold a false belief rather than a true one.

• On the covariationist reading, Gettier cases result from a

failure of the belief in p, the truth of p, and the evidence E for

believing p to covary in close possible worlds.

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• Smith‘s reason for holding the belief true have nothing to

do with why the belief is true.

• On the identificationist reading, Gettier cases result from

a failure of S‘s reasons for holding the belief true to

identify the belief‘s truth-maker.

• Covariation failure in close possible worlds vs.

identification failure in the actual world.

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