Overview – Knowledge from Reason

This A Level philosophy topic examines 2 ways we can acquire knowledge through reason, i.e. a priori:

and considers the kinds of things we can know through these methods (the debate between rationalism and empiricism).

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

Some definitions

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

Rationalism / empiricism

The debate between rationalism and empiricism concerns the relationship between the two kinds of knowledge (a priori/a posteriori) with the two kinds of truth (analytic/synthetic).

Most a priori knowledge is knowledge of analytic truths and most a posteriori knowledge is knowledge of synthetic truths. The disagreement between rationalism and empiricism is whether this relationship holds in all cases, or whether there are exceptions:

  • Rationalism says there are some synthetic truths that can be known a priori
  • Empiricism says all knowledge of synthetic truths is acquired a posteriori

In other words, empiricism says there is no such thing as synthetic a priori knowledge and rationalism says there is at least one synthetic truth that is known a priori.

Intuition and deduction

The first way we might acquire knowledge a priori is through intuition and deduction. These faculties can be thought of as the ability to see truth. For example:

  1. If it it Monday then I have philosophy class today
  2. It is Monday
  3. Therefore, I have philosophy class today

The reason you can see that 3 logically follows from 1 and 2 is via rational intuition. You don’t need someone to explain why 3 follows from 1 and 2, you just see it.

Descartes: Meditations

Using only these faculties of intuition and deduction, Descartes attempts to establish various synthetic truths. If his arguments work, then they support the rationalist claim that some synthetic truths can be known a priori.

The three waves of doubt

In his attempt to establish knowledge, Descartes begins by doubting 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.

Descartes Evil Demon Scepticism

This position of extreme doubt is known as 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 and the external world

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

His ontological argument is 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. Everything that exists has a cause
  3. Therefore, my concept of God must have a cause
  4. The cause of an effect must have at least as much reality as the effect
  5. My concept of god contains perfection
  6. Therefore the cause of my concept of God must contain perfection
  7. No being which is not God contains perfection
  8. Therefore, the cause of my idea of God is God
  9. Therefore, God exists

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

Descartes then continues his process of intuition and deduction to argue that because God exists, his perception can be trusted. Therefore, the external world 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

Another way we might acquire knowledge a priori is through innate knowledge. Innate knowledge is knowledge you’re born with and so doesn’t require experience to be known.

Plato: Meno

Meno's slave square examplePlato argues that we are born with certain kinds of knowledge. The key facts of Plato’s argument for innate knowledge are listed below:

  • Meno’s slave has never been taught geometry, so he doesn’t know geometry
  • Socrates draws a square on the ground that is 2 feet x 2 feet
  • Socrates asks Meno’s slave a series of questions
  • Meno’s slave correctly answers the questions
  • This leads him to realise the area of the square is 4 feet, despite having no knowledge of geometry

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

Tabula rasa

John Locke famously argues that the mind at birth is a tabula rasa – a blank slate. He says all knowledge and concepts come from experience and therefore there is no such thing as innate knowledge.

Abstract general ideas

Locke’s theory of abstract general ideas explains how humans can form concepts, such as God, without invoking innate concepts.

He says we form the concept of trees, for example, by abstracting the common features from individual trees – leaves, roots, branches, etc. Similarly, we form the concept of beauty by abstracting from experience. We see a beautiful lake, a beautiful painting, a beautiful person, etc. and these experiences lead us to form the abstract concept of beauty. Locke claims all concepts, such as trees, beauty, and God, are derived from experience and so are not innate.

Arguments against innate knowledge

Locke argues that if we did have innate knowledge, such as Plato claims in the Meno’s slave example, such knowledge would be universal. But, he argues, children and ‘idiots’ do not know the theorems of geometry. So, this knowledge is not universal and therefore not innate.

Locke gives two further reasons to reject innate knowledge:

  • Observation of newborn babies suggests they do not have any concepts beyond those experienced in the womb (e.g. 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. Not only that, there are entire societies who do not have the concept of God

However, we may respond (as Leibniz does) that it’s possible to have innate concepts, yet not be conscious of them. This would explain the two examples above.

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