KRIPKE ON THE A PRIORI
Kripke: philosophers have shared the
following assumptions:
A priori knowledge is of necessary truths.
 E.g.: Chisholm, Kant and Ayer.
 Quine’s denial of necessary truths is a
denial of the a priori.
All a posteriori knowledge is of contingent
truths.
 E.g.: Chisholm, Kant and Ayer.
 Quine: all knowledge is revisable.
Kripke thinks that both principles are
mistaken.
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Names
Old Theory (Frege-Russell): Names are
abbreviated descriptions:
E.g.: Aristotle was Plato’s greatest student.
 So: ‘Aristotle’ means ‘Plato’s greatest
student’.
Kripke: If ‘Aristotle’ means ‘Plato’s greatest
student’, then:
 ‘Aristotle might not have been Plato’s
greatest student’ is a contradiction!
But Aristotle might not have been Plato’s
best student or even his student at all.
 He might have gone into Politics,
become a soldier, raised horses, etc.
 These are not contradictions.
The old theory is, therefore, implausible.
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Identity
True: Aristotle might not have been a
student, or Plato’s student, or a philosopher.
 These are contingent features of
Aristotle
False: Aristotle might not have been
Aristotle.
Why? Try to imagine a situation (possible
world) in which Aristotle exists, but is not
Aristotle.
 If that person is not Aristotle, then it is
not a case in which Aristotle is not
Aristotle.
 If that person is Aristotle, then it is not a
case in which Aristotle is not Aristotle.
Conclusion: Necessarily, A = A.
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Rigid designators
X is a rigid designator if in every possible
world it designates (refers to) the same
object.
The above reasoning tells us that ‘Aristotle’
is a rigid designator:
 Since there is no possible world in which
Aristotle exists, but is not Aristotle, it
follows that ‘Aristotle’ picks out the same
individual in every world.
Kripke: names are rigid designators.
 ‘Aristotle’ picks out or refers to a
particular individual person.
 If we want to consider what could have
been true of him, we must imagine him
but with different properties.
Names pick out individuals.
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Names and necessity
Suppose ‘Aristotle’ refers to a particular
person.
 Suppose ‘Eltotsira’ refers to that same
person (different language).
We know that:
 Necessarily, Aristotle = Aristotle.
 Necessarily, Eltotsira = Eltotsira.
But we know that each name simply picks
out one and the same individual.
 We also know that that any individual is
necessarily self-identical.
So:
 Necessarily, Aristotle = Eltotsira.
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The argument summarized
1.
2.
3.
So:
4.
Suppose A = B.
Necessarily, A = A.
Necessarily, B = B.
Necessarily, A = B (substitute (1) into
(2) or (3)).
Conclusion: If A = B, then necessarily A = B
(where A and B are names).
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A Historical example
Suppose you are taught that ‘Hesperus’
refers to that bright, morning sky object.
 You are also taught that ‘Phosphorus’
refers to that, bright night sky object.
 All your life you believe: ‘Hesperus is the
morning star and Phosphorus is the
evening star’.
Later on, Astronomers perform some
observations. They tell you that:
 The morning star and the evening star
are just Venus.
 I.e. ‘Hesperus’ refers to Venus.
 ‘Phosphorus’ refers to Venus.
Since Hesperus = Phosphorus, necessarily
Hesperus = Phosphorus.
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Empirically discovered necessities
Kripke: that Hesperus = Phosphorus was an
empirical discovery.
 Astronomical observations were
required to learn it.
 So, it is a posteriori.
However: what is learned is necessary.
So, some claims are necessary but a
posteriori.
Other necessary a posteriori claims:
 Water = H20
 Temperature = mean kinetic molecular
energy.
 Goldbach’s Conjecture?
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Contingent a priori
Suppose I stipulate:
 The length, L, of this bar in front of me is
the standard measure for ‘one metre’.
What follows from this?
 I know that the bar in front of me is one
metre long.
 I don’t have to measure anything—this
is known a priori.
But is this claim necessary?
Kripke: No.
 If it were necessary, it would be
impossible for the bar to be any other
length.
But this is not impossible:
 If the bar were hotter when I’d made my
stipulation, it would have been a
different length.
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Fixing the reference
When I make my stipulation, I want to pick
out or refer to a certain length (= 39.37 in.)
I use the property ‘length of this bar’ in order
to pick out this length.
 In other words, this property fixes the
reference of ‘one metre’.
Kripke: it does not follow that this property is
essential or necessary for the bar in
question.
 Whatever length the bar has is
contingent: it might have been
otherwise.
 Still, one metre = length L remains
necessary (rigid designation).
Conclusion: some claims are a priori but
contingent.
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Conclusion
The a priori/a posteriori distinction does not
track the necessary/contingent distinction.
Necessary
Contingent
A priori
‘All bachelors
are
unmarried’
‘This bar is
one meter
long’
A posteriori
‘Hesperus is
Phosphorus’
‘John is 6’
tall’
Synthetic a priori knowledge is possible
(contra Ayer).
Synthetic a priori knowledge does not
require that the world conform to the mind
(contra Kant)
A priori knowledge is possible even if there
are no analytic claims (contra Quine).
A priori Knowledge is not limited to
necessary claims (contra Chisholm).
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Kripke’s vision
If water = H20, then necessarily water = H20.
 But this claim is not necessary in virtue
of the meanings of words.
 It is not part of the meaning of ‘water’ or
‘H20’ that they be identical.
This is a kind of essentialism: water has a
necessary property: being H20.
 Anything that is not H20 is necessarily
not water even if it is liquid, colourless,
tasteless, etc.
What do you think: are there some features
of objects or people that are necessary in
this way?
 Could something be water even if it had
molecular structure XYZ?
 Is this an unduly strong commitment to
current chemical theory?
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