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.

Necessary / contingent

The distinction between necessary and contingent truths is very similar to the distinction between analytic and synthetic truths:

  • Necessary truths must be true (so are more or less analytic truths)
    • E.g. “Triangles have three sides”
  • Contingent truths might not have been true (so are more or less synthetic truths)
    • E.g. “Toblerone is a triangle shape”

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 looks at whether we can acquire knowledge of synthetic truths using a priori reasoning (intuition and deduction):

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

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. 

Descartes, for example, is a rationalist because he says we can learn synthetic truths such as “God exists” and “I exist” purely through intuition and deduction i.e. without empirical experience.

Innatism / empiricism

The debate between innatism and empiricism asks whether humans have any innate knowledge, i.e. knowledge that they are born with.

  • Innatism says there is some knowledge that is innate knowledge.
  • Empiricism says there is no such thing as innate knowledge and that all knowledge is acquired after we are born.

Plato, for example, is an innatist and tries to show with his slave boy example that even without education, humans have knowledge of geometry.


Qualia are the subjective qualities of experience – i.e. what something feels like inside your mind. Qualia are not properties of objects, but properties of minds/experience. Examples of qualia include:

  • The redness I experience when I look at a ripe tomato
  • The taste of beer when I have a drink
  • The rough feeling when I run my hand over some sandpaper

Knowledge of qualia is sometimes called phenomenal knowledge – i.e. knowledge of what it is like to have a certain experience.

Local / global scepticism

Scepticism is about doubting certain knowledge claims. We can distinguish between two different levels of scepticism

  • Local/ordinary scepticism is when you doubt ordinary things in everyday life.
    • E.g. “I doubt whether he was the killer” or “I doubt that England are going to win the World Cup”
  • Global/philosophical scepticism is when you extend doubt beyond these normal levels and doubt pretty much everything that is possible to doubt.
    • E.g. “I might be dreaming, in which case I should doubt everything I am currently looking at” or “I might be a brain in a vat, so I can’t know for certain that today is Monday”

The classic global sceptic’s position is illustrated by Descartes in his 3 waves of doubt

In ordinary circumstances, for example, it doesn’t make sense to doubt whether you have hands (assuming you do and are looking at them). But if you’re in a philosophical setting, you can doubt you have hands because you can imagine scenarios where you would believe you have hands even when you didn’t. For example, you might actually just be a brain in a vat being stimulated by electricity to believe it has a body and hands etc. Or reality might be completely different and you actually have tentacles but an evil demon is deceiving you to believe you have hands instead. It doesn’t seem likely, but it’s philosophically possible.