Rationalism
Theory of knowledge
Instructions
Use this PowerPoint presentation as you would any other; press F5 on your keyboard to start
presenting and use the arrow keys to go backwards or forwards.
License
Licensed under Creative Commons BY-NC-SA 2.0 UK: England & Wales
Version
This means you’re free to i) Copy, distribute & perform the work and
ii) Make derivative works, as long as you i) Attribute the work (and any
1
derivate works) to Pisp.co.uk, ii) Do not use it for commercial gain and
iii) License any derivative works under an identical license. More...
2008-10-17

This PowerPoint presentation is based on an article on the
Pisp.co.uk website.
For more information, on this subject or many others, visit
Pisp.co.uk and click Learn. You’ll also find links to
download this show in PowerPoint format.
LEARN MORE ONLINE...

All the stuff created by the Pisp.co.uk team is licensed
under Creative Commons where possible.
We encourage you to print or pass this resource on to
others who might find it useful! If you edit, you agree to
license your edited version under the same license and
not to use it for commercial gain.
SHARE THIS...

Fast facts
• Rationalism is from the Latin word ratio
meaning reason
• Rationalists believe we can have
knowledge of certain a priori principles
that are the basis of the world
• The flip-side is empiricism, though most
rationalists don’t totally reject empiricism
• Prominent rationalists include Plato and
Descartes

Welcome
• Rooted in Ancient Greek philosophy
dating back to Plato and Zeno
• It lay pretty dormant until the 17th and
18th centuries, when it was bought back to
the forefront by Descartes, amongst others
• In our science-driven society the idea of
thinking through problems rather than
inductively working them out through
experimentation seems a little alien

What is rationalism?
• Michael Lacewing and Jean-Marc Pascal:
“Rationalists argue that it is possible to
know some synthetic propositions about
how the world outside of the mind is
without using sense experience.”
Example
If you toss a coin up into the air, the coin will come
back down. How do you know? Rationalists hold
that we have knowledge of certain a priori
principles, like the laws of physics, that are the
basis for the world.

Rationalism = reason!
Source of
knowledge Justification for
our beliefs
Source of
Reason conceptual
apparatus

‘Armchair philosophers’
• Not accurate since all Rationalists (bar
Plato) acknowledge that everyone needs a
bit of sense experience sometimes
 We need to be equipped with training in
language and logic (i.e. maths) to be able
to rely solely on reason
 Particular facts can only really be
induced (a posteriori), for example the
distance from an examination hall you once
sat in to the examiner’s house where your
paper was marked

However, the rationalist
maintains that basic facts of
reality can be deduced
using our reason alone

However, the rationalist
maintains that basic facts of
reality can be deduced
using our reason alone
Example
To know the time now we would need to
check a clock. To understand the nature
of time, we need no apparatus; we have
to do is to reason the concept of time.

How does it work?
• Rationalists argue that either...
We have a We have knowledge of
Rational Certain innate
‘intuition’ truths
that allows us to grasp which are part of our
certain basic truths rational nature

Knowledge via reason
• According to rationalists, knowledge via
reason is superior because:
 Innate knowledge is superior
Everything is in a state of flux; Things are
dying, things are being born – there’s change
 Can we trust this changing world?
 If we’re to rely on our reason, we’re immune
from change since the laws of maths or
physics simply aren’t subject to change

Knowledge via reason continued
• According to rationalists, knowledge via
reason is superior because:
 Some knowledge is restricted to reason
Some knowledge is only accessible by
reason
 E.g. maths

Enter: Zeno
• Ancient Greek philosopher
• Stands in the shadows of the big guys –
Plato, Aristotle and Socrates
• Consider...
• “In a race, the quickest runner can never
overtake the slowest, since the pursuer
must first reach the point whence the
pursued started, so that the slower must
always hold a lead.”

Achilles vs. The tortoise
1. A tortoise and Achilles, a Greek hero,
are having a race
2. Achilles, being nice and all, gives the
tortoise a head start; the tortoise plods
along at a slow, but constant speed
3. To catch up, Achilles first has to get to
where the tortoise originally started
and, by that time, the tortoise has
moved on

Achilles vs. The tortoise continued
4. So now Achilles gets to where the tortoise
has moved on to, but the tortoise has
moved on again
5. So Achilles runs to where the tortoise has
moved onto again, but the tortoise has, of
course, moved on from there, too
6. And so on...
• Does Achilles ever catch up?

Achilles vs. The tortoise continued
4. So now Achilles gets to where the tortoise
has moved on to, but the tortoise has
moved on again
5. So Achilles runs to where the tortoise has
moved onto again, but the tortoise has, of
course, moved on from there, too
This is the wrong question to
ask; this wasn’t the point Zeno
6. And so on... was trying to make...
• Does Achilles ever catch up?

What’s the point
• But people do overtake
others in races!
• That’s not the point,
the point is that we’ve
learnt that time and
space cannot be
divided into infinitely small parts
• We did it all using reason; we didn’t go
out, collect Achilles and a tortoise and pit
them against each other in a race

Empirical and logical truths
• What constitutes knowledge by reason
and knowledge by sense-experience?
• Empirical and logical truths categorise
the two
• Empiricism is another word for sense-
experience, empirical knowledge is
knowledge gained through experience

Empirical and logical truths continued
Empirical truths Logical truths
• Concerns truths that • These are necessary
could have been • There is no possible
otherwise way that a logical truth
• Contingent, meaning could ever be (or have
they rely upon something been) otherwise
else and that they can, • Up is not down is a
and do, change logical truth
• A pre-historic duck that • Contrariwise, up is down
was thought to be extinct, is a logical falsity.
for instance, turns out to
be thriving in a remote
part of the Congo

Empirical and logical truths continued
Empirical truths Logical truths
• The flag of the European • This room is red, the flag
of the EU is blue – both
Union consists of 12 stars
arranged in a circle empirical truths
• Could have been • But, red is not blue is a
arranged in a square logical truth

Plato
• Another Ancient Greek
philosopher
• Plato rejected all sense
experience
• For him, the problem of the
one and the many meant
that empiricism was useless
• The problem of the one
and the many...

No man ever steps in the
same river twice, for it's not
the same river and he's not
the same man.

The problem of the one and the many
• Put forward by Heraclitus, quoted by
Plato in Cratylus
• The world is in a state of flux; it is
constantly changing
• Water in a river continuously flows, it’s
not the same water as before
• The man stepping is constantly aging;
new cells are being developed, old ones
are falling off etc.

So, what’s the problem?
• Knowledge gained from a state of flux
leads to knowledge that is itself flawed
• Instead, Plato deduced that we ‘recollect’
knowledge from ex-temporal (outside of
time) abstract entities called ‘forms’
• Each object or concept in this world has a
correlating form in ‘the realm of the forms’
• The forms are immune to the
constrains of this world, like decay, so
they are perfect in every way

So, what’s the problem?
• Knowledge gained from a state of flux
leads to knowledge that is itself flawed
• Instead, Plato deduced that we ‘recollect’
knowledge from ex-temporal (outside of
time) abstract entities called ‘forms’
• Each object or concept in this world has a
correlating form in ‘the realm of the forms’
• The forms are immune to the
constrains of this world, like decay, so
they are perfect in every way

So, what’s the problem?
• Knowledge gained from a state of flux
leads to knowledge that is itself flawed
• Instead, Plato deduced that we ‘recollect’
knowledge from ex-temporal (outside of
time) abstract entities called ‘forms’
• Each object or concept in this world has a
Allow Bertrand Russell
to clarify... realm of the forms’
correlating form in ‘the
• The forms are immune to the
constrains of this world, like decay, so
they are perfect in every way

There are many individual animals of whom we can
truly say „this is a cat‟. What do we mean by the
word cat? Obviously something different from each
particular cat. An animal is a cat, it would seem,
because it participates in a general nature common
to all cats. Language cannot get on without general
words such as „cat‟, and such words are evidently
not meaningless. But if the word „cat‟ means
anything, it means something which is not this or
that cat, but some kind of universal cattiness. This is
not born when a particular cat is born, and does not
die when it does, in fact, it has no position in space
or time; it is „eternal‟.
(Bertrand Russell History of Western Philosophy. London: Routledge, 2004.)

Clear and distinct ideas
• That’s Plato, jump into the future a few
thousand years and we’ll meet Descartes
• He wanted to reach an indubitable truth
by asking the question (in French)...
What is it possible to doubt?
(A-level French was totally worth it!)

Clear and distinct ideas
• He wanted to
know what gave
certain truths
their certainty
• Set up criteria for
establishing what
exactly makes a
truth self-evident:
the clarity and
distinctness...

Clearing things up (pun)...
• Clear ideas
An idea is clear if
we cannot help but
take notice of it
• So, strong physical
sensations, like
pain, or thoughts
like the desire for
something
Desire for fruit!

Distinct ideas
• Distinct ideas
An idea is distinct if it
cannot possibly be
confused with anything else
• Think about a time you've
had a headache; the pain is
clear, but it's indistinct since
you don't know exactly
where the pain is or what's
causing it

Final thoughts
• So far we’ve just talked about rationalism
from various philosophers including Plato
and Descartes
• You’ve probably thought of a bunch of
criticisms but save them for next time
because empiricism is rationalism’s
biggest criticism
• Tune in next time for empiricism!