Nicky asked:

How does Plato’s Theory of Forms relate to rationalism, how does it relate to empiricism, how does it relate to Kant’s theory?

Answer by Helier Robinson

Plato’s theory of forms postulates a second world, beyond the empirical world, such that the empirical world is a poor copy of the world of forms. This is often called the two-world hypothesis, and is invoked to explain illusions in the empirical world: illusions are poor copies of reality and we know them to be such because they are either empirical contradictions (e.g. the half immersed stick is bent to the sight and straight to the touch) or else they contradict well-established belief (e.g. the railway lines do not really meet in the distance). Contradictions are impossible (except in language) and hence illusions are unreal. Plato was mostly concerned with the conflict between Heraclitus and Parmenides, which arose from the fact that if you apply logic strictly to common sense you find that one thing cannot change with time and remain one. So for Heraclitus only change is real, and for Parmenides only the One is. Plato’s world of forms is Parmenidean: the forms are unchanging and perfect. And the empirical world is largely illusory, including the illusion of change.

Rationalism goes along with Plato. Rationalist philosophy begins with the fact of illusion and the need to both explain it and correct it. This is done with the representational theory of perception, in which real objects cause images (representations) of themselves in the brains of perceivers; in so far as the images are untrue to their originals, so are they illusions, or misrepresentations. This theory is bolstered by the fact that every empirical object, if you think carefully about it, is a structure of sensations; but all sensations are manufactured in the brain, so that the empirical world is inside your head, and this seems absurd since it is obvious that all empirical objects are outside our heads.

Herein lies the basic disagreement between rationalists and empiricists.

Empiricists side with common sense and say that the empirical world is real, in the sense of existing whether perceived or not; but they cannot satisfactorily explain illusions or the sensational character of empirical objects. Rationalists can explain these, but seemingly at great cost to common sense. This was resolved by Leibniz, who realised that if every empirical thing that we perceive is an image of reality, rather than reality itself, then our own empirical bodies are images and so we have two bodies (as well as two worlds): they are the noumenal body and the empirical body. (I use the word noumenal here because the word real is so ambiguous; it means known by the mind, as opposed to the empirical, which means known by the senses.) Consequently your entire empirical world (which is always limited by horizons of the moment, such as the blue sky) is contained in your noumenal head. So beyond the blue sky is your noumenal skull. Rationalists are very rare these days in philosophy departments but common in physics departments, where they are called theorists. Theoretical science describes the underlying causes of empirical things, and these underlying causes are noumenal. (But I do not know of any theorists who take this to its logical conclusion and put their noumenal skulls beyond all horizons of the moment.)

Kant had two theories. In his youth he was a Leibnizian and wrote of two worlds, noumenal and phenomenal (i.e. empirical), but later, after he read David Hume, who ‘awoke him from his dogmatic slumbers,’ he tried to reconcile empiricism and rationalism. He failed. Like Hume, he finished up on the slippery slope to solipsism, with nothing but dogmatic common sense preventing him from sliding all the way down. And like Leibniz he retained two worlds but insisted that the noumenal world was radically unknowable, and he failed to recognise Leibniz’ distinction between our noumenal and phenomenal bodies.