Overview – Knowledge from Reason

This A Level philosophy topic examines whether all our knowledge comes from perception or whether there are other – a priorisources of knowledge. There are really two separate debates in this topic, and these are:

Broadly speaking, empiricism says that all knowledge comes from experience. Innatism and rationalism are two ways we can reject this claim:

  • Rationalism says that we can acquire some knowledge purely through intuition and deduction (i.e. we can acquire knowledge purely by thinking rather than through perceptual experience).
  • Innatism says that we are born with some knowledge already (and innate knowledge obviously doesn’t come from perceptual experience because you haven’t had any perceptual experience when you are newly born).

This topic also touches on whether it is possible to know anything at all (scepticism).

Some definitions

A lot of the discussion (in particular rationalism vs. empiricism) draws on the following terms.

Analytic / synthetic

Analytic and synthetic are two different kinds of truths.

  • Analytic truth: true in virtue of the meaning of the words
    • E.g. “A bachelor is an unmarried man”, or, “triangles have three sides”
  • Synthetic truth: true in virtue of how the world is
    • E.g. “Grass is green”, or, “water boils at 100°c”

Analytic truths cannot be denied without resulting in a logical contradiction. To say, “not all bachelors are unmarried”, for example, is to misunderstand the word bachelor – the concept of a married bachelor does not make sense. Similarly, one cannot coherently imagine a triangle with four sides because the very idea involves a contradiction.

Denial of a synthetic truth does not lead to a logical contradiction. For example, we can coherently imagine red grass in denial of the synthetic truth “grass is green”. Though experience tells us grass is not, in fact, red, there is no logical contradiction in this idea.

A priori / a posteriori

A priori and a posteriori are two different kinds of knowledge:

  • A priori knowledge: knowledge that can be acquired without experience of the external world, through thought alone
    • E.g. working out what 900 divided by 7 is
  • A posteriori knowledge: knowledge that can only be acquired from experience of the external world
    • E.g. doing an experiment to discover the temperature at which water boils

Intuition and deduction

Intuition and deduction are a priori methods for gaining knowledge:

  • (Rational) intuition: The ability to know something is true just by thinking about it
  • Deduction: A method of deriving true propositions from other true propositions (using reason)
    • E.g. You can use deduction to deduce statement 3 from statements 1 and 2 below:
      1. If A is true then B is true
      2. A is true
      3. Therefore, B is true

In the overview above, we defined rationalism as the view that we can acquire some knowledge purely through intuition and deduction. This is slightly inaccurate, though, because even an empiricist would admit there is some knowledge that can be known purely through intuition and deduction. For example, you clearly don’t need empirical experience to work out that “2×75=150” because it is an analytic truth. So actually, what empiricists say is that you can’t acquire knowledge of synthetic truths using intuition and deduction:

  • Empiricism says all a priori knowledge is of analytic truths (i.e. there is no synthetic a priori knowledge)
  • Rationalism says not all a priori knowledge is of analytic truths (i.e. there is at least one synthetic truth that can be known a priori using intuition and deduction)

Most of the time, empiricism holds true. If you take any synthetic truth, such as “water boils at 100°c”, it seems impossible that we could learn it without some a posteriori experience of the world (e.g. an experiment).

So, most synthetic truths are known a posteriori. But the question is whether this relationship holds true all the time or just some of the time. Rationalism says the latter: There is at least one synthetic truth that can be known a priori through intuition and deduction.

Descartes: Meditations

Descartes’ Meditations provides arguments for 3 synthetic truths using a priori intuition and deduction. The synthetic truths Descartes argues for are:

  • “I exist”
  • “God exists”
  • “The external world exists”

If Descartes’ arguments for these claims work and are purely a priori, then they support the rationalist claim that some synthetic truths can be known a priori.

The three waves of doubt

Before seeking to establish what he can know, Descartes first seeks to doubt everything he thinks he knows. If it’s possible to doubt it, then he does.

These sceptical arguments have come to be known as the three waves of doubt. They are:

  • Illusion
  • Dreaming
  • Deception

First off, Illusion. I can doubt the reliability of my sense experience as it has deceived me in the past. For example, a pencil in water may look crooked even though it isn’t.

Second, I might think I’m awake when I’m actually dreaming. I might believe I’m looking at a computer screen, but if I’m simply dreaming that I am, then my belief is mistaken.

But even if I’m dreaming, there are still basic ideas that are common to both dreams and reality. For example, that “1+1=2” – can this be doubted?

Yes, says Descartes, if I am being deceived. An evil demon may be controlling my entire experience, making me think I’m correctly adding 1 and 1 when I’m not.

So, basically anything I think I know can be doubted – an evil demon may be controlling my perception and making me have nothing but false beliefs. The evil demon scenario could be true, and there is no way I would be able to tell the difference. So, the possibility of the evil demon scenario casts doubt on everything I know.

Descartes Evil Demon Scepticism

This position of extreme doubt is known as global scepticism.

Cogito ergo sum

So, Descartes is currently in a position of extreme scepticism. Is there anything he can know for certain?


dubito, ergo cogito, ergo sum

This translates as:

  • I doubt
  • Therefore I think
  • Therefore I am

The shortened version – cogito ergo sum – is probably the most famous phrase in philosophy. Descartes’ point is that even if the evil demon is trying to deceive him on just about everything, Descartes can not doubt that he exists.

The reason for this is that even if the demon is deceiving him, there must be something for the demon to deceive in the first place! The fact that Descartes is able to doubt his own existence is proof that he does, indeed, exist.

Clear and distinct ideas

Upon reflection of the cogito ergo sum argument, Descartes claims that his certainty in the proposition ‘I exist’ is due to the fact that it is a clear and distinct idea. Descartes knows ‘I exist’ is true simply by thinking about it.

This property – where the truth of an idea presents itself clearly and distinctly – goes back to the idea of rational intuition.

Rational intuition is the faculty that allows you to see truth. This faculty is what allows Descartes to know ‘I exist’ simply by thinking about it.

Arguments for the existence of God

Descartes builds on these clear and distinct ideas via deduction. He gives several arguments for the existence of God using intuition and deduction:

His ontological and cosmological arguments are covered in the philosophy of religion module.

A summarised version of the trademark argument is as follows:

  1. I have the concept of God
  2. My concept of God is the concept of something infinite and perfect
  3. But I am a finite and imperfect being
  4. The cause of an effect must have at least as much reality as the effect
  5. So, the cause of my concept of God must have as much reality as what the concept is about
  6. So, the cause of my idea of God must be an infinite and perfect being
  7. So, God exists

This summary misses out a few steps, but you get the idea.

This argument is called the ‘trademark’ argument because Descartes argues that concept of God is like an innate ‘trademark’ placed in our minds. Descartes’ point is essentially that it’s impossible this trademark – of an infinite and perfect being – is something he could have created himself. Whatever caused Descartes’ idea of God must itself be an infinite and perfect being because of premise 4 above.

Argument for the existence of the external world

Descartes then continues his process of intuition and deduction to argue that because God exists, his perception can be trusted. These perceptions are of an external world of physical objects and, because Descartes can trust his perceptions, he can trust that the external world exists.

The key points are as follows:

  1. I have perceptions of an external world with physical objects
  2. My perceptions cannot be caused by my own mind because they are involuntary (Descartes’ argument for this is similar to e.g. Locke’s argument here)
  3. So, the cause of my perceptions must be something external to my mind
  4. God exists (see trademark argument above)
  5. If the cause of my perceptions is God and not the physical objects themselves, then God has created me with a tendency to form false beliefs from my perception (because premise 1)
  6. But God is a perfect being by definition (see e.g. Descartes’ ontological argument) and so would not create me with a tendency to form false beliefs from my perceptions
  7. So, I can trust my perceptions
  8. So, given premises 1 and 7 above, I can know that an external world of physical objects exists


One way we can challenge the trademark argument is by rejecting the first premise and arguing that the concept of God is not innate. If the concept of God comes from experience, as Locke argues, then Descartes argument is not a priori and thus fails to establish rationalism.

Hume’s Fork

David Hume (1711 – 1776) was a knowledge empiricist. He argued that there are only two kinds of knowledge:

  • Relations of ideas (i.e. analytic, a priori)
  • Matters of fact (i.e. synthetic, a posteriori)

If it is not a relation of ideas or a matter of fact, says Hume, it is not knowledge.

Relations of ideas

A relation of ideas is:

“either intuitively or demonstratively certain”

In other words, an analytic truth, and:

“discoverable by the mere operation of thought, without dependence on what is any where existent in the universe”

In other words, known a priori.

Like analytic truths, relations of ideas cannot be denied without a contradiction (e.g. “that triangle does not have 3 sides” is a contradiction).

Matters of fact

Hume says:

“The contrary of every matter of fact is still possible; because it can never imply a contradiction”

In other words, matters of fact are synthetic truths. Also, they require empirical observation to establish their truth, so are known a posteriori.

Innate knowledge

Innate knowledge is knowledge you’re born with and so doesn’t require experience (a posteriori) to be known. In other words, innate knowledge is a priori knowledge.

The debate between rationalism and innatism is about whether we have innate knowledge or not:

  • Innatism says we have some innate knowledge
  • Empiricism says we do not have any innate knowledge

It’s also important to note that the kind of knowledge we are considering in this innate knowledge discussion is innate propositional knowledge. It’s uncontroversial, for example, that babies are born with innate ability knowledge, such as knowing how to breathe.

Plato: Meno

Meno's slave square examplePlato argues that all learning is a form of recalling knowledge from before we’re born. So, in other words, we’re born with innate knowledge and we just need to remember it.

To prove his theory, Plato shows how Meno’s slave – a boy who has never been taught geometry – is able to understand a geometry proof. The key facts of the argument are as follows:

  • Socrates draws a square on the ground that is 2 feet x 2 feet
  • Meno’s slave agrees its area is 4 square feet
  • Socrates then draws another square on the ground that has an area of 8 square feet
  • Socrates then asks: What are the lengths of the sides?
  • Meno’s slave incorrectly guesses 4 feet initially (the area would be 16 square feet, not 8)
  • But Socrates asks Meno’s slave a series of questions
  • Meno’s slave answers the questions correctly and realises that the sides of a square with an area of 8 square feet will be equal to the diagonal of the original 2 feet x 2 feet square

If this doesn’t make sense, the 18 second video below is a much easier way of visualising Socrates’ proof. Notice that the green square is twice the area of the red square and that the sides of the green square are equal to the diagonal of the red square.

Again, Meno’s slave has never been taught geometry, i.e. he has no experience of geometry. Despite this, Meno’s slave is able to correctly answer Socrates’ questions (or at least correct his mistakes). Given that Meno’s slave has no experience of geometry, his correct knowledge here must be innate.

Leibniz: Necessary Truths

A posteriori experience can only tell us about specific instances. For example, experience can tell us that adding these 2 apples to these 2 two apples gives us 4 apples.

Such experience tells us how things are, but not how they must be. In fact, no amount of experience can tell us how things must be (because even if you conduct 100000 experiments, you never know what will happen in experiment 100001).

Yet we do know that it must always be true that “2+2=4”. We know that there will never be an instance where you add 2 apples to 2 apples and get 5 apples. In other words, “2+2=4” is a necessary truth.

We know this by paying close attention to “what is already in our minds”. In other words, Leibniz is saying that knowledge of necessary truths is innate.


John Locke: Essay Concerning Human Understanding

Arguments against innate knowledge
1. Innate knowledge would be universal

Locke argues that if we did have innate knowledge then every human would have such knowledge.

So, for example, everyone would know the theorem of geometry that Meno’s slave realises in Plato’s example above. But, Locke argues, children and ‘idiots’ do not possess such knowledge – they don’t know the theorems of geometry, for example. So, this knowledge is not universal and therefore not innate.

2. Argument against innate concepts

Locke also argues against the existence of innate concepts. The argument is that propositional knowledge relies on concepts. For example, you can’t know that “1+1=2” without first having the concepts “1” ,“+”, and “2”. So, if Locke succeeds in disproving the existence of innate concepts, he will also succeed in disproving the existence of innate knowledge.

Locke gives two examples to reject the existence of innate concepts:

  • Observation of newborn babies suggests they do not have any concepts beyond those experienced in the womb (e.g. the concepts of warmth or pain)
  • God is often used as an example of an innate concept (as seen in Descartes’ trademark argument) but babies do not have this concept. Further, there have been many atheist societies throughout history that did not have the concept of God.

Possible response to Locke’s arguments against innate concepts:

Leibniz agrees with Locke that innate knowledge requires innate concepts. But Leibniz argues that it’s possible to have innate concepts and yet not be conscious of them.

Some of Leibniz’s own examples of innate concepts are logical concepts such as identity (e.g. “a = a”) and impossibility (e.g. “it’s impossible for both a and not a to be true”). Obviously, a newborn baby can’t verbally articulate these thoughts, but this doesn’t mean the concept isn’t there. We innately know these concepts – even if we can’t articulate them – and they are essential to all thought, whether we consciously recognise them or not. Over time, we learn to recognise these concepts and make them explicit, but they were always there in the mind.

Similarly, just because some people and societies may lack a word for ‘God’, this doesn’t mean they lack the concept. It may take experience to consciously develop the concept of God, but the concept itself can’t come from experience because it goes beyond experience. For example, the concept of God is the concept of an infinite being, but nothing in experience shows us this concept of infinity.

Tabula rasa

It follows from Locke’s rejection of innatism that all knowledge (and concepts) must come from experience. Locke famously argues that the mind at birth is a ‘tabula rasa’ – a blank slate. Locke argues that the mind at birth contains no ideas, thoughts, or concepts. Instead, knowledge comes from two types of experience:

  • Sensation: Our sense perceptions – what we see, hear, smell, taste, etc.
  • Reflection: Experience of our own minds – thinking, wanting, believing etc.
Simple and complex ideas

Locke’s explanation of simple, complex, and abstract general ideas provides an account of how humans can form complex concepts, such as the concept of God, from experience.

When I look at a clear sky, my sensation of blue might give me the simple concept of blueness. Likewise, when I’m outside in winter, my sensation of cold might give me the simple concept of coldness. A simple concept is just one thing like this.

Complex concepts are made up of the building blocks of simple concepts. For example, my concept of the ocean could consist of both the simple concepts of blue and cold above. Pretty much everything is a complex concept made up of simple concepts to differing degrees – for example a chair might consist of many simpler concepts (e.g. brown, hard, wooden, small, etc.) and yet they all form the same thing: This chair. This chair is a complex concept because it contains many simpler concepts within it.

And complex ideas can go beyond specific instances of things (e.g. this chair) to form abstract ideas (e.g. chairs in general). For example:

  • Chair #1 may be wooden and have four legs
  • Chair #2 may be metal and have 3 legs
  • Chair #3 may be a red plastic stool

Similarly, we form abstract concepts such as beauty or justice by abstracting from experience. For example, we see a beautiful lake, a beautiful painting, a beautiful person, etc. and over time we abstract the common features from these experiences to form the abstract concept of beauty. Locke claims all concepts – from the simple to the complex – are derived from experience in some way, and so are not innate.

<<<Knowledge from Perception

The Limits of Knowledge>>>