A response to the skeptic
Phil 2233, Fall 2007
Some things I know about the past
• I had scrambled eggs for breakfast this
morning.
• John F. Kennedy was assassinated in
1963.
• Glaciers spread across northern Europe
and North America within the last 20,000
years.
• Trilobites were arthropods that thrived
during the Paleozoic era.
Justifying such claims
• I remember eating the eggs
• I remember JFK’s assassination and that it took place
during the year I was in grade 2, i.e. in 1963.
• I’ve directly seen evidence of glaciation (striations,
polished pavements, kettle lakes, moraines, drumlins,
etc.) and I’ve also read a lot about the evidence, its
history, and the conclusions geologists have drawn from
it.
• I’ve seen fossils of trilobites and read about them as
well. Their place in the geological column is securely
settled (they only occur in Paleozoic strata), and they are
classified as arthropods because they have certain
features that are shared with, and only with, other
arthropods.
The skeptic’s response
• Someone defending a general skepticism
about the past looks for some assumption
that plays a role in all these justifications, a
vulnerability that they all share.
• And they do have something in common:
They all depend on assumptions about
processes linking facts about the present
situation to some fact(s) about the past.
Historical evidence
• When it comes to documented historical events
a number of different processes are involved.
Events occur; some people perceive them,
report them, and they are subsequently
recorded in various kinds of documents.
• These documents may persist into the present,
or we may only have copies of copies of copies.
But so long as the processes involved in making
those copies reliably preserve reliable
information originally encoded in the first
records, we can reach conclusions about the
past by reading these records.
Geology
• The same goes for geological evidence: processes take
place during glaciation, when an organism is fossilized,
as sediment is laid down, etc. These processes leave
traces that persist[1] over time.
• Then we can find out about glaciation, past life and etc.
by studying the traces that exist today.
• In general, a present trace encodes information about a
past event because of a process linking that event and
certain facts about it to the present existence and
features of the trace.
•
[1] Note that persisting is really just another kind of
process.
The Skeptical Argument
• Processes take place over time. So any knowledge we
have about processes depends on having knowledge
about the past.
• To justify my knowledge claims about the past, I need
more than just knowledge of the present traces. I also
need knowledge of the processes that produced them.
• So I need knowledge of the past to justify knowledge
claims about the past.
• We are stuck in a circle—to justify a claim about the
past, we must already know something about the past.
So if our knowledge about the past must be founded
exclusively on knowledge about the present, we can’t
have any knowledge of the past at all.
Radical skepticism about the past
• No matter what we now observe, we can’t justify
inferring anything about what has happened
before.
• In particular, from this point of view nothing rules
out the infamous ‘five minute hypothesis’, which
holds that the entire world came into existence
only five minutes ago, with all our ‘memories’,
‘historical records’, ‘fossils’ (both in the ground
and in the museums), and other traces of the
‘past’ just as we find them to be.
Comments on the 5-minute
hypothesis
• We cannot prove that this silly story is fiction using only
our present observations.
• Without claims about past processes to draw on, we
can’t make any inferences from present evidence to
conclusions about the past.
• The five minute hypothesis is logically compatible with
our present evidence, including the evidence we
normally regard as clearly demonstrating certain facts
about the past. The five minute hypothesis blocks all our
common-sense inferences about the past. From this
perspective, present evidence imposes no constraints at
all on what might have happened in the past.
The Criterion
• Skeptical circles like this are familiar puzzles. They go
back to an ancient argument called ‘the wheel,’ also
known as the problem of the criterion.
• The wheel argument is far more general than our puzzle
about the past: It asks, “how can we have knowledge of
truth?”
• We do take certain claims to be true, and we reject other
claims as false; if someone asks us how we decide what
claims are true (and what claims are false), we must
describe some method(s) we use to separate these
truths and falsehoods. Let’s suppose that the methods
we come up with really do support the claim we’ve
made.
The trap
• Next, our questioner asks how we know that our criterion
is a good one. This simple question puts us in an
awkward position!
• We can say that there is some higher-order criterion (a
criterion for good criteria) that this criterion meets. But
then our questioner will ask how we know this new
criterion is a good one, and we’re off on a regress that
seems to be endless. Or we can say the criterion is a
good one because it picks out sentences we know are
true. But then the skeptic points out that we invoked the
criterion in the first place to justify our choice of
sentences. Therefore it’s circular for us to appeal to our
sentences to justify the criterion. At this point it seems
all we can do is to look dazed and sorry.
Back to the past
• It seems that we cannot justify our claims about
processes without appealing to facts about what has
happened in the past. Further, we cannot justify claims
about the past without some knowledge of the processes
linking present evidence about the world to past facts
about the world.
• The result of taking such arguments to be conclusive is
wholesale skepticism, and wholesale skepticism is very
unattractive. Far better to find a response to the
skeptic’s challenge that preserves a healthy common
sense account of what we know and what we don’t.
Resisting the skeptic
• Resistance begins with a diagnosis of where things have
gone wrong.
• If we assume that we start with no idea at all of what is
true or how to tell truth from falsehood, then the problem
of the criterion is serious trouble.
• If we have no idea what’s true or how to tell if a sentence
is true, then there’s nothing for us to go on. In fact, from
this empty point of view we have no idea of what ‘true’
(or ‘the past’) might mean, and no idea of how to find
out!
• But you can’t find something out unless you have some
idea of how to tell when you’ve got it right. You can’t
even begin when you have no idea what you’re looking
for.
A better approach
• We’re better off if we start with a reasonable list of things
that we think are true, and some criteria for truth.
• Then we have something we can build on: We can
compare the things we take to be true with what we get
by applying the criteria we have to decide what’s true
and what isn’t.
• We can improve on our criteria by considering the results
of applying various criteria and examining how they fit
with what we take to be true. And we can improve on
what we think is true by applying our criteria and
adjusting our beliefs to fit the results of applying them.
Applying this to the past
• Similarly, we can study processes as they occur,
and study the detailed physics and chemistry
involved in them as well. And we can compare
this information against what we already believe
about the past.
• Over time this allows us to extend and improve
the fit between our criteria and our beliefs: We
come to describe ourselves, our methods and
their results in rich and detailed ways that fit
together to support the conclusion that by using
these methods we can reliably know these this
about our world.