Philosophy counsels us to begin with the most certain of definitions. In that spirit, returning to a question I've often avoided, what is the common definition of knowledge between the rationalists and empiricists? Their work concerns how we accumulate knowledge, but what is it that we accumulate? Justified true belief, in the classical mold? Is there a common definition between the two traditions at all?

Generally, what is the common ground between the rationalist thinker and his empiricist counterpart--but to be more precise, how can we define or recognize knowledge, the accumulation of which is in dispute? One side says that it is innate; the other that it is derived from experience--but what is it?

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    Don't have time to answer in sufficient detail but the core answer is that each is committed to the idea there is a sure foundation for knowledge in a particular source ... ergo they are both foundationalist epistemologies even if they disagree about whether reason or the senses supplies the foundation.
    – virmaior
    Mar 8, 2018 at 23:56
  • But what is their criteria for knowledge? Using empiricism as an example: the senses can provide various data: what an object looks like, what a texture feels like... how particles interact, how weather occurs. The idea of knowledge isn't exhausted by any of these determinations; nor is it necessarily limited to them. We don't know an object in the same way we do a person, or a scientific law. Mar 9, 2018 at 2:05
  • Knowledge for these foundationalists is unassailable access to reality as it is.
    – virmaior
    Mar 9, 2018 at 7:04

1 Answer 1


Where there is disagreement, as there is between rationalists and empiricists, there is always a background of agreement : there can only be disagreement if you at least agree on what there is to disagree about. So much by way of sage introduction !


I'd offer a provisional characteristisation of rationalism as a philosophy for which reason is a source of knowledge of the physical world independent of and superior to empirical observation in the form of sense perception or introspection. 'Superior to' indicates that reason can supply beliefs which are immune from error. A rationalist accepts the possibility and fact of a priori knowledge, knowledge derived by reason independently of (prior to) experience. Reason may operate independently of experience by virtue of innate ideas but a rationalist is not as such committed to the existence or reliability of innate ideas.

There has historically been an assumption, or tendency to suppose, that knowledge of physical world takes a mathematical form.


For empiricism there is no knowledge independently of experience. The closest we can come to a priori knowledge is a knowledge of analytic truths - knowledge for instance of tautologies. This tells us nothing about reality, only at most some truths about language or concepts. All knowledge of the physical world derives from empirical observation in the form of sense perception or introspection.

Empiricism is not closely associated with the view that a knowledge of the physical world takes a mathematical form.


I have only been able to offer a wide and approximate contrast between rationalism and empiricism. There is really not way round looking at individual thinkers and seeing what they agreed and differed on.


☛ Direct perception and certainty

The 'reason' on which rationalism relies is not necessarily or even usually deductive, inferential, 'discursive' reasoning - the deduction of a conclusion from premises. It is often, as we see clearly in Descartes, rational intuition or insight. Rationalists use deduction but Descartes' 'Meditations' and to an extent his 'Rules for the Direction of the Mind' run on intuitions - the products of mental acts by which we perceive directly and with absolute objective certainty*, without the aid of empirical observation, some particular truth.

[* Certainty is ambiguous between (psychological) 'absolute conviction' and (epistemological)'immunity from error'. I use the term in the second sense.]

Direct perception also plays its part in empiricism but not perception of the intuitive kind. If, for instance, it appears to me that I am looking at a tree, then it is certain that that is how things appear to me. Even if I am hallucinating, it is still certain that it appears to me that I am looking at a tree. Without further analysis, Descartes regarded such empirical certainty as too thin to support the edifice of knowledge; Meditation I shows how sense-based certainty can go along with epistemological unreliability. How things appear, whatever the fact of our experience, is no sure guide to how they really are.

☛ Individualism

In Descartes on the one hand on the rationalist side, and on the other Locke and Hume on the empiricist, there is a kind of epistemological individualism. Locke is careful, or tries to be, to admit no truth which does not recommend itself to his own mind. Descartes, equally, commits himself to not accepting as true anything which he does not clearly apprehend to be such. Hume keeps closely to beliefs which he can justify in terms of his own 'impressions and ideas'.

☛ Rationalism does not exclude the empirical totally

Observation, sensation, and experiment - which clearly belong to the world of experience - are by no means excluded from the Cartesian search for knowledge. Descartes never says that we or he can build an adequate body of knowledge without any reference to the senses. It is just that the contribution of the senses is either unnecessary or highly unreliable at the deepest, foundational level of knowledge. The foundations of knowledge are supplied by clear and distinct ideas derived from intuition working in the 'natural light' (lumen naturale) of reason. There is plenty of room for observation, sensation, and experiment but only insofar as their results can be justified by derivation from or consistency with foundational clear and distinct ideas.

  • But is there a common definition, in the same sense that antiquity could say that knowledge is justified true belief? The knowledge of the statement "I think" present in the texts of Descartes is distinct from the basic sense-impressions of Hume. Mar 10, 2018 at 2:08
  • The idea that knowledge is justified true belief is a gloss on a passage in Plato's Theatetus. It was not the view of Antiquity but of one thinker, Plato. Aristotle does not offer this definition of knowledge. Also I accept that there is a contrast - there are many contrasts - between Descartes and Hume but you asked specifically not for differences or distinctions but for points in common. I indicated a number of such commonalities, which is what you asked for. You might consider framing another question if you want to draw out the differences. You put a good question. I tried to answer it.
    – Geoffrey Thomas
    Mar 10, 2018 at 3:59

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