Topics in Epistemology
Sven Rosenkranz
Introduction
One of the central questions of philosophy is to what extent we can be confident that we
can properly identify our relation to the reality of which we are only a small part. There
are two aspects to the question.
On the one hand, we may ask how our subjective experience can be conceived to be part
of the objective reality which we confront, and thereby inquire whether we are able to
arrive at a coherent picture of the latter that includes both the painter and the painter’s
point of view. This question lies at the heart of the philosophy of mind.
On the other hand, we may ask what it takes for us to accurately represent how things
objectively are, how we can assure ourselves that we succeed in our attempts to
accurately represent them, and how far our ability to do so extends. This is the province
of epistemology.
We can distinguish three questions that epistemology is concerned to answer:
 What is the nature of knowledge?
 What are the sources of knowledge?
 What is the extent of our knowledge?
In this course, we will chiefly be concerned with the third of these questions, which we
will discuss in the context of the recent debate about scepticism.
1. The nature of knowledge
Knowledge implies belief: if I know that p, then I take it to be the case that p (i.e. I take
reality to be such that it is not misrepresented by saying that p). To take it to be the case
that p is to believe that reality is as one thereby takes it to be, and so to believe that p.
So we get:
(i)
Kxt[p]  Bxt[p],
where ‘x’ ranges overs subjects and ‘t’ ranges over times. Although belief is necessary
for knowledge, it is not sufficient for knowledge. One may (and frequently does) hold
beliefs that do not correspond to how things really are, i.e. one may (and frequently
does) believe that p, for some proposition p, although it is not the case that p. By
contrast, one cannot know that p unless it is the case that p. So we get:
(ii)
Kxt[p]  p.
We may put this by saying that knowledge implies truth.
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Does the combination of these two necessary conditions suffice for knowledge? In other
words, is true belief sufficient for knowledge? No. In order for a belief to count as
knowledge it is not enough that the belief be true. It is further necessary that the belief
be formed in appropriate ways – that it is, in some sense, responsive to the truth.
For instance, if you come to believe that the person you just met is honest with you by
consulting the stars, then even if that person is indeed honest with you, you do not know
on that basis that she is. And if you come to believe that you have been selected by the
recruitment committee because you know you have been lucky in the past, then even if
you have in fact been selected, you still do not know on this basis that this is so. In some
pertinent sense, your beliefs in these cases are no better than mere guesswork.
It is a matter of debate what it takes to form a true belief ‘in appropriate ways’ in order
for that belief to constitute knowledge. But we may at least say this much:
(iii)
Kxt[p]  at t x has come to believe that p by successful application of a truthconducive method.
The term ‘method’ is used in a broad sense to cover any kind of cognitive procedure, or
routine, whose implementation can coherently be thought of as a means to find out
whether p.
A method is truth-conducive only if its successful application leads to the formation of
beliefs that are more likely to be true than false. One way to flesh out the latter thought
is by saying that a method is truth-conducive only if it is reliable.
Note that successful application of a reliable method does not guarantee the truth of the
belief to whose formation it leads. The reliability of a method is not a matter of its
always leading to true beliefs, nor of its leading to a true belief at all times at which it is
correctly classified as reliable. Rather, its reliability requires it to be such as to yield
mostly true beliefs. Consequently, its (present) reliability is a matter of how it works in a
whole range of cases (present, past and future).
However, just as a meter reliably indicates a given quantity only insofar as it operates
under conditions that don’t impair its function, a method will be reliable only relative to
the prevailing of favourable environmental conditions. For instance, looking at a
middle-sized object’s surface is a reliable means to find out which colour this object has
only in circumstances in which the lighting conditions are normal. Similarly, checking
on whether the water in the pot is boiling is a reliable means to find out whether the
water’s temperature is around 100 degrees Celsius only if one is close to sea-level.
So when we here speak of successful application of a method we mean to secure that
two rather distinct kinds of conditions are met:
(a)
the subject is in the right state of mind (is lucid, sober, attentive etc.), performs
well, properly responds to the outcome and, in general, does all she is expected
to do in order to correctly apply the method
(b)
the prevailing environmental conditions are favourable, i.e. of the kind relative
to which alone the method can count as reliable.
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As just said, the consequent of (iii) does not guarantee the consequent of (ii): a subject
may successully apply an otherwise reliable method, so that the environmental
conditions are favourable and the subject does all she is required to do in order to
rationally arrive at a belief on that basis, and yet the subject’s belief may be false. Thus,
for example, checking how y looks is a pretty reliable means to find out whether y is a
duck, but for all that, objects on occasion are not what they look and if confronted with
a fake duck on a pond using that means will deliver a false belief.
Some philosophers have imposed further conditions on knowledge that are meant to
close this gap. Here are two such further conditions:
Sensitivity:
(iv)
Kxt[p]  the method m by which x has come to believe that p at t is such that if
p were false and x were to apply m, x would not come to believe that p on that
basis.
The italicised clause is a subjunctive conditional. The truth-conditions for subjunctive
conditionals can be understood as follows:
A  B  The nearest A-worlds are B-worlds.
Whether a world is nearby or remote is somewhat vague. It depends on the extent of
change required in order for that world to be actual (or correspondingly, on how similar
that world is to the actual world). In any case, if A holds in the actual world, then
‘A  B’ implies that B holds in the actual world, because then the actual world is
amongst the nearest A-worlds. It follows that the subjunctive conditional ‘A  B’
entails the material conditional ‘A  B’.
Requirement (iv) is called ‘sensitivity’ because it gives expression to the idea that the
method employed must be sensitive to whether p holds.
Safety:
(v)
Kxt[p]  the method by which x has come to believe that p at t is such that if x
were to believe that p on the basis of m, p would be true.
Requirement (v) is called ‘safety’ because it embodies the idea that the method
employed does not easily lead one astray.
A method of belief formation may be safe and yet fail to be sensitive. For example, I
may come to believe, truly as we might suppose, that Mireia is not a political radical by
gathering the kind of evidence one usually has for such claims (observations of her
behaviour, testimony etc.); and yet, if Mireia was a political radical, she would be wise
enough to conceal this, given her circumstances, and so I would still come to believe,
albeit falsely, that she is not a political radical on that basis. So my method isn’t
sensitive. But given that Mireia is in fact not a political radical, but a sober and
moderate thinker whose sobriety and moderateness is deeply anchored in her upbringing
and education, worlds in which she is a political radical are rather remote, at least they
are more remote than the nearest possible worlds in which I come to hold the same
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belief on the basis of the same kind of evidence. So my method, though insensitive, is
safe all the same.
Each of (iv) and (v) entails that if at t x comes to believe that p on the basis of
successfully applying method m, x’s belief is true. That this is so can be shown as
follows, where ‘Bxtm[p]’ abbreviates ‘At t, x believes that p on the basis of applying m’
and ‘Axtm’ abbreviates ‘At t, x has applied m’.
1
2
2
1,2
5
5
1,2,5
2,5
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
¬p
Bxtm[p]
Axtm
¬p & Axtm
(¬p & Axtm)  ¬Bxtm[p]
(¬p & Axtm)  ¬Bxtm[p]
¬Bxtm[p]
p
assumption
assumption
from 2
from 1 and 3
Sensitivity
from 5
from 4 and 6
from 2 and 7
1
2
2
1,2
(1)
(2)
(3)
(4)
Bxtm[p]
Bxtm[p]  p
Bxtm[p]  p
p
assumption
Safety
from 2
from 1 and 3.
Typically, those who think that (iv) imposes a legitimate constraint on knowledge do
not say the same about (v), and those who think that (v) imposes a legitimate constraint
on knowledge do not say the same about (iv). This is an ongoing debate.
Are the conditions mentioned so far jointly sufficient for knowledge? There is the
concern that they are not. For note that even if a subject has come to hold a belief by
successful application of a reliable method which is safe (or sensitive), the subject need
not be aware of, let alone possess evidence for, these features. Thus, for all that has so
far been said, the subject may not be aware of, or have evidence for, any of the
following:




the method used is reliable
the environmental conditions under which this method is applied are favourable
if p were false and one were to apply the method, one wouldn’t come to believe that p
if one were to believe that p on the basis of applying the method, p would be true.
These conditions are at least as external to the subject’s mind as the state of affairs to
which ‘p’ relates. And doesn’t this just show that more is required of knowledge than
that these conditions are satisfied?
After all, the principal reason why we strive for knowledge, and are not content with
mere true belief, is that we wish to get some reassurance that our representations of
reality are accurate. Carefully following through a cognitive procedure by applying a
given method in a lucid, sober and attentive way, and forming beliefs in response to the
outcome of this application, does not yet guarantee that the method is as a matter of fact
reliable and safe (or sensitive) and that the environment does as a matter of fact
cooperate. Yet, if these are the external conditions distinctive of knowledge, how can
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the desired reassurance be obtained by their simple fulfilment, while we so far have
done nothing to ascertain that they are fulfilled?
Those who take these concerns very seriously, and to disclose that we so far haven’t
specified sufficient conditions for knowledge, are the epistemological internalists. By
contrast, epistemological externalists rest content with the conditions specified thus far.
The dispute between internalists and externalists is sometimes said to turn on the status
of the so-called KK-principle:
(KK) Kxt[p]  Kxt[Kxt[p]],
where ‘p’ ranges over non-epistemic states of affairs. Note that without some such
restriction of the range of ‘p’, the KK-principle would already be psychologically
implausible, because then the K-operator would be said to iterate indefinitely – just plug
in the consequent into the antecedent’s ‘p’-slot. Since knowledge implies belief, any
knowing subject would have to be credited with infinitely many beliefs. Other
principles of iterativity are not subject to this constraint, because they use epistemic
notions that do not imply belief, e.g.:
Kxt[p]  x is in a position to know that Kxt[p]
Kxt[p]  it is possible for x to know that Kxt[p].
If the KK-principle holds, then it will not do that the environmental conditions are as a
matter of fact favourable and that the method used is de facto reliable (and safe or
sensitive): in order for a subject to have knowledge it is furthermore required that the
subject knows that this is so.
However, although they typically deny the KK-principle, externalists qua externalists
need have no qualms with any of this, and the internalists’ worries cannot thereby be
allayed, if to have such second-order knowledge is again a mere matter of forming
beliefs by applying a de facto reliable (and safe or sensitive) method under de facto
favourable conditions.
A better way to draw the line is to introduce a decidedly internalist notion of evidence.
In order to do so, let us first reflect on what is involved in applying a method and to
form a belief in response to the outcome of this application.
Consider the method of using one’s senses to tell which colour a given middle-sized
object has. Looking in the direction of the object and focusing on its surface, we are led
to have a certain experience as of this object’s being, say, red. We form the belief that
the object is indeed red, and do so in response to this experience. If the object had had a
different colour, say, blue, we would have been led to have a qualitatively distinct
experience, viz. as of its being blue, and would consequently have formed the belief that
it is blue (and not red). Similarly, if we measure a certain quantity by means of a meter,
e.g. the pressure in a cooker, we first take note of the meter reading and then form the
corresponding belief about the pressure inside. If the meter reading had been different,
so would have been our belief.
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In both cases, we gain evidence for our respective beliefs (experiences in the first case,
meter readings in the second). And now the important point is that we would not come
to acquire knowledge in this way if we did not take note of this evidence. And even if
the evidence itself is external to the subject’s mind (as in the case of the meter
readings), taking note of this evidence, possessing it, is not (but is as internal to the
subject’s mind as the colour experience).
We may now formulate a principle about which internalists and externalists are likely to
disagree:
(*)
There is no necessary condition for knowledge for whose obtaining the knowing
subject possesses no evidence.
Internalists will accept (*), while externalists will reject it.
For the externalist, (*) sets the bar too high, because for the externalist, whether a given
piece of information can really count as evidence for p will inter alia depend on its
pedigree (its origin or the way it was obtained); and it is unlikely that any evidence will
ever ‘wear its pedigree on its sleeves’. Rather, the right kind of pedigree that makes for
evidence is the pedigree of information which is made available by successful
application of a reliable method under favourable conditions; and we cannot tell
whether a given piece of information has this feature just by taking it in. Yet, the idea
that knowing subjects possess evidence, evidence for the right pedigree of that evidence,
evidence for the right pedigree of the latter and so on and so forth, is, if these pieces of
evidence are distinct, just as psychologically implausible as the unrestricted version of
the KK-principle that allows knowledge to iterate indefinitely.
Let us follow Crispin Wright and call pieces of evidence, on the externalist construal of
‘evidence’ according to which a piece of information counts as evidence only if it has
the right pedigree, ‘warrants’. The notion of a warrant is partly internalist in that
possessing evidence is a state of mind; and it is partly externalist because having the
right pedigree requires that condition be met that relate to matters outside the subject’s
mind. Since, as we have noted, the consequent of (iii) does not entail the consequent of
(ii), one may possess a warrant for a belief, although this belief is false.
We may use the notion of warrant to define an internalist notion of evidence which we
may call ‘reason’:
x has a reason for p  x is in a state subjectively indistinguishable from
possessing a warrant for p.
To possess a warrant for p is to have a reason for p, since possessing a warrant for p is
indistinguishable from itself. But the other direction does not hold, because reasons may
lack the right kind of pedigree.
From a subjective perspective, the notion of reason is more central than that of warrant,
precisely because the subject cannot, just by processing the information she has,
distinguish between being in a state in which she possesses a warrant for p and being in
a state that is merely subjectively indistinguishable from a state of the first kind.
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Is (*) too demanding if ‘evidence’ is understood in terms of reasons?
Ignoring the sensitivity and safety requirements for the moment, we must check what it
would take for a subject to possess reasons for the following external constraints:
 the method used is reliable
 the environmental conditions under which this method is applied are favourable.
So far we haven’t distinguished between all-things-considered evidence (warrants,
reasons) and prima facie evidence (warrants, reasons). Prima facie evidence for p can
be overruled by countervailing evidence but still counts as prima facie evidence for p
once so overruled, and it will be all-things-considered evidence for p at a given time, if
no such countervailing evidence comes to light at that time. Note, though, that even allthings-considered evidence can be overruled in the sense that there may be all-thingsconsidered evidence for p at one time, but no such evidence at another time. We may
think of prima facie evidence for p as pieces of information that may or may not add up
to all-things-considered evidence for p at a given time depending on the other
information available at that time. Rationally formed beliefs respect all-thingsconsidered evidence (rather than merely prima facie evidence).
In light of this distinction, our previous description of what application of a method
involves needs to be refined. For example, inspecting an object’s surface in order to tell
which colour it has will lead us to enjoy a certain experience as of that surface’s being a
certain colour, but whether we form the belief that it has that colour will also depend on
the other evidence we have at the time. Thus, if we also have evidence to the effect that
there are artificial sources of coloured light illuminating the object’s surface, we may
well refrain from forming the relevant belief.
Of course, that further evidence may not have the right pedigree. But even if we have no
direct knowledge of the pedigree of the evidence we have accumulated, we can tell
whether it coheres. Coherent evidence accumulated over long periods of time by using
different techniques or sense-modalities may put us in a position to draw an inference
to the best explanation concerning its origins. As the case may be, a world in which p
is the case and the method we apply is reliable and is applied under favourable
circumstances may be the best bet in order to account for the overall evidence we have
so far accumulated. It is in this kind of way that an internalist may accept (iii) as a
constraint on knowledge and yet comply with (*), on her preferred construal of
‘evidence’ in terms of reasons.
2. The sources of knowledge
When it comes to the second epistemological question, there are two principal
distinctions we need to draw, that between a posteriori and a priori knowledge and that
between inferential and non-inferential knowledge.
A posteriori knowledge is knowledge based on experience, whereas a priori
knowledge is not so based. There may be a priori knowledge that p even if we have to
have had some experience in order to acquire the concepts involved in the proposition
that p. The terms ‘a posteriori’ and ‘a priori’ do not relate to the genesis of our
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knowledge, but to its rational grounds (that which we would cite in order to justify our
corresponding beliefs).
A paradigmatic example of a posteriori knowledge is knowledge by perception (outer
sense). It is our visual, auditory or tactile experience that is the source of our perceptual
knowledge. A paradigmatic example of a priori knowledge is knowledge by
mathematical proof. Here the source of our knowledge is not our outer sense, but
reasoned reflection. Of course, we may also prove certain conclusions based on
premises furnished by observation. But if the proof of these conclusions really depends
on these premises, then the knowledge of them which we thereby acquire is not a priori
but a posteriori, even if the proof rules we use can be recognised to be valid on purely a
priori grounds.
Inferential knowledge is based on inference, whereas non-inferential knowledge is
not so based. A paradigmatic example of non-inferential knowledge is again knowledge
by perception. Thus, when we see what the meter reads, we do not infer what it reads,
even if we go on to infer from the meter reading what the pressure inside the pressure
cooker is. A paradigmatic example of inferential knowledge is again knowledge by
mathematical proof.
However, although mathematical knowledge by proof is a paradigmatic case of both
inferential and a priori knowledge and knowledge by perception is a paradigmatic case
of both non-inferential and a posteriori knowledge, a posteriori knowledge may be
inferential and a priori knowledge may be non-inferential. For instance, given our
understanding of the concepts involved, we may come to know a priori that any
president presides over something, without drawing any inference in the process.
Similarly, we may come to know a posteriori that sometimes partnerships aren’t
harmonious by inferring it from the known premise that Sun and Jin quarrel.
Also, we cannot determine by the content of a piece of knowledge alone what its source
must be: knowledge that the sum of 2 apples and another 3 apples is 5 apples may be
arrived at a priori by use of basic arithmetic but it may also be arrived at a posteriori by
counting apples. Similarly, one may know non-inferentially that someone is next door
(by hearing them quarrel), but may equally know this indirectly, by inference from
certain clues.
Non-inferential knowledge may be divided into knowledge by outer sense (perceptual
knowledge) and knowledge by inner sense. Knowledge by inner sense may be further
divided into knowledge by introspection (of our mental lives), knowledge by
understanding (of conceptual truths) and knowledge by rational intuition. The latter
is arguably a category under which nothing falls, but some philosophers have insisted
that it covers our knowledge of some parts of mathematics.
Inferential knowledge can be further divided into knowledge by deduction and
knowledge by induction, corresponding to the divide between deductive and inductive
inference. While deductive inferences necessarily preserve truth, and so help to discern
the logical consequences of what we know, inductive inferences are ampliative in that
the truth of their conclusions go beyond what the truth of their premises logically
guarantees.
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Examples of deductive inferences are:
No human has scales
The creature from the black lagoon has scales
-----------------------------------------------------------The creature from the black lagoon is not human
This is a pen
Pens exist outside our minds
------------------------------------------Something exists outside our minds
Examples of inductive inferences are:
All observed amphibians lay eggs
-----------------------------------------All amphibians lay eggs
The sun has always risen in the past
--------------------------------------------The sun will rise tomorrow
A special case of inductive inferences are abductive inferences (inferences to the best
explanation). Examples are:
Otterbourne was killed with a gun
Bourget was seen at the murder site
Bourget had a motive
Bourget’s finger prints were on the gun
------------------------------------------------Otterbourne was killed by Bourget
It appears to me as if there was a pen lying
in front of me
I feel lucid
The lighting seems normal
----------------------------------------------------There is a pen lying in front of me
As said, unlike deductive inferences, inductive inferences are not necessarily truthpreserving, i.e. there are possible circumstances in which their premises are true and
their conclusion is nonetheless false. This immediately raises the question of why they
should nevertheless be considered good inferences on which we can rationally rely
when forming our beliefs. One may think that inferences of this kind – e.g. the inference
concerning tomorrow’s sunrise – have proved reliable in the past and so should be
trusted now. However, this kind of reasoning does itself use an inductive inference, viz.
the one that proceeds from the observation that in the past the inference did not lead us
astray to the conclusion that it will not lead us astray now – which is no better or worse
than to infer from the fact that the sun has always risen in the past that it will rise
tomorrow. Accordingly, the worry that inductive inferences are in bad standing has not
been allayed. This is the famous problem of induction, first clearly stated by David
Hume.
Deductive inferences are not beset by this problem and so there are no corresponding
qualms about whether it is rational to rely on them in forming our beliefs. Still, it needs
to be explained in what sense they are at the service of inferential knowledge; and here
philosophers typically invoke the following principle of closure:
(Closure)
(Kxt[p] & (p ⊢ q) & at t x has competently deduced q from p)  Kxt[q],
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where ‘A ⊢ B’ says that A entails B. We may assume that in order to competently
deduce q from p, the subject must know at that time that p entails q. But this knowledge
is in any case not enough in order to guarantee that if the subject knows that p, she
thereby comes to know that q: she must further perform the deductive inference in
question and thereby arrive at the belief that q.
As we shall see later, Closure, although widely accepted, is not uncontroversial.
However, here I just want to highlight the contrast between Closure and another, closely
related idea, viz. that deductive inferences are at the service of acquiring evidence for
the conclusion. In the literature, this idea goes by the name ‘warrant transmission’,
because the evidence, if any, that a competent reasoner acquires for the conclusion will
be the evidence she has for the premises which accordingly transmits across the
(known) entailment.
Closure may hold, and yet a deductive inference may, at least on occasion, fail to
prescribe a route by which evidence may be acquired for the conclusion. Here is an
example:
Suppose my evidence for the claim
A caravan of camels passes by some miles ahead
is that it visually appears to me as if a caravan of camels passes by some miles ahead.
Suppose that I then reason as follows:
A caravan of camels passes by some miles ahead
If a caravan of camels passes by some miles ahead, then what I see is no mirage
--------------------------------------------------------------------------------------------------What I see is no mirage.
The inference is deductively valid, and we can safely assume that the second premise is
well-established, given my circumstances. But do I thereby acquire evidence that what I
see is no mirage, evidence that is transmitted from the premises to the conclusion?
Hardly. For, it seems that unless I could already discount beforehand that I am the
victim of an illusion, I cannot take the evidence I have for the first premise to be
sufficient to license my endorsement of that premise. Yet, if I could already discount
beforehand that I am the victim of an illusion, then I had evidence for the conclusion
already before I drew the inference, evidence that is in no way strengthened by the
evidence I have for the first premise.
Does this example likewise show that Closure fails? No. For if I really do know the first
premise on the basis of my visual evidence, so that I am entitled to regard that evidence
as sufficient to license my endorsement of it, then I must have independent evidence
that puts me in a position to know the conclusion.
What the example does show, however, is that the validity of Closure, though
necessary, is not sufficient to explain how we can attain knowledge by deductive means.
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3. The extent of our knowledge
The question of how far our knowledge extends is not intended as a question about the
extent of the knowledge we have so far acquired. Rather, it asks about the extent to
which we can have knowledge. Anti-Realists hold that every truth is knowable, apart
from those truths that record our ignorance, e.g., as the case may be, ‘The number of
leaves on the Sycamore tree in my garden is even but no one will ever know’. Setting
aside such truths (which are already unknowable for logical reasons), Anti-Realism is a
fairly optimistic thesis about the reach of our epistemic powers which the Realist denies
or at least considers unjustified.
Realism seems a reasonable stance. However, denial of Anti-Realism, or its rejection as
unjustified, can be understood to rest on rather different grounds. This becomes more
obvious once we distinguish between kinds of claims that we endeavour to have
knowledge of. Examples of such kinds are:








:
claims about the colour, shape and location of middle-sized everyday objects
claims about the nature of middle-sized objects (whether they are cats, pens or rocks)
claims about the microphysical properties of microphysical objects
claims about one’s own mental life
claims about the mental lives of others
claims about lawlike connections between events
claims about the past
claims about the future
Denial of Anti-Realism is compatible with the idea that for each such kind, we can
know some claims of that kind. Similarly, rejecting Anti-Realism as unjustified is
compatible with the concession that we are justified in thinking that we know some
claims of each such kind. Thus, the position we commonly associate with the label
‘realism’ holds that although we can and do know claims of each of these kinds, and are
justified in thinking that we can and do know this, we cannot know, or at least do not
know that we can know, all truths of these kinds.
Scepticism, by contrast, questions our capacity to know any claim of a given kind.
Thus, scepticism about the past challenges our confidence that we are able to know
anything at all about the past. Scepticism about other minds challenges our confidence
that we are able to know anything at all about the mental lives of others. The radical
sceptic challenges our confidence that we are able to know anything at all about the
external world, not even that there is an external world!
In this course, we will be concerned with this radical form of scepticism.
The sceptical challenge may take the shape of a flat-out denial that we can have the
relevant knowledge (first-order scepticism) or, more guardedly, the denial that we have
any justification at all to think that we can have that knowledge (second-order
scepticism).
Sceptical challenges are developed by means of argument. Keith DeRose gives a
general characterisation of first-order sceptical arguments, which I will reproduce here:
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Let O be an everyday claim about our environment which we ordinarily take ourselves
to know, e.g.




that one has hands
that there is a pen lying in front of one
that one’s mother feels sad
that one had breakfast this morning.
For dialectical reasons, O should be thought of as a best case, i.e. as a claim of a kind
(about one’s physical environment, about other minds, about the past) such that if one
does not know that claim one does not know any claim of that kind.
Let H be some kind of sceptical hypothesis, either to the effect that some cognitively
disabling conditions obtains, e.g.
 that one is dreaming
 that one is an envatted brain
 that one is manipulated by an evil demon
or to the effect that reality is fundamentally different from what we take it to be, e.g.
 that there is no external world
 that are no other minds
 that reality came into existence 5 minutes ago replete with traces of an apparent past.
DeRose then takes the template for sceptical arguments to be this:
(1)
(2)
¬Kxt[¬H]
¬Kxt[¬H]  ¬Kxt[O]
Therefore:
(3)
¬Kxt[O].
Premise (2) is equivalent to: Kxt[O]  Kxt[¬H]. Accordingly, if O is known to entail
¬H, then premise (2) will be derivable from Closure, provided only that x can be
assumed to competently perform the relevant deduction. Arguably, all of the following
entailments hold:
I have hands ⊢ There is an external world
My mother feels sad ⊢ There are other minds
I had breakfast this morning ⊢ Reality didn’t come into existence 5 minutes ago.
If this is known to be so and x competently deduces the right-hand sides from the lefthand sides, then Closure allows us to conclude that if x fails to know the right-hand
sides, x likewise fails to know the left-hand sides.
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Given how O was chosen, x will then fail to know any ordinary claim about her
physical environment, any ordinary claim about the minds of others and any ordinary
claim about the past.
Note that Closure alone will not necessarily furnish us with premise (2) if H is a
sceptical hypothesis about a cognitively disabling condition. Thus, the following
entailments plausible do not hold:
I have hands ⊢ I am not dreaming/being manipulated
My mother feels sad ⊢ I am not dreaming/being manipulated
I had breakfast this morning ⊢ I am not dreaming/being manipulated.
The case of envatment is rather different. Thus, if I am an envatted brain then I don’t
have hands, and if I was always an envatted brain, then I won’t have a mother and most
definitely didn’t have breakfast this morning.
In light of such sceptical arguments, one has in principle three options: one may either
succumb to scepticism and accept (3), or one may deny (1) or may deny (2). As we shall
see, contextualism offers a more subtle line of response. It remains to be seen whether
any of these responses is satisfactory and if so which is more satisfactory than the
others.
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