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.
Ask a Philosopher 
Plato’s Forms are abstract high level ideals of notions such as 1) virtue, justice, wisdom, and 2) equality, likeness, unity, and 3) heat, cold, tallness, and 4) hair, clay, dirt (from the Parmenides). Forms evolved in Plato’s writings from Socratic vagueness in the early dialogues to Parmenidean oneness in the Phaedo to logically relational objects, ‘beings’, in the Sophist and the Philebus.
Forms are described in the Divided Line (the Republic) as metaphysically distinct objects of thought that are separate from illusional sensibles and particular objects, and also separate from logical and mathematical structures.
This complexity created a number of important, as yet unresolved philosophical puzzles of particular objects and participation of these objects in the Forms. The motivation is that neither thought nor communication are held possible without some version of Plato’s Forms and related particulars.
The relation between the four parts of the Divided Line was never un-puzzled by Plato. However, he was working on resolving these metaphysically transcendent problems throughout the late dialogues.
Although Plato himself never formulated any such thing, the Line, the Cave, and the problem of participation are often taken to be a ‘Theory of Forms’.
So now, to reformulate the original question, rationalism, empiricism, and Kant are attempts to develop or to improve on Aristotle’s philosophy. Aristotle held particular objects to be real in a reversal of Plato’s preference for the permanent world of ideas. This is already well described by the preceding answer and comment.
I do not take Kant to have failed, slippery slope to solipsism? And I have a doubt, as to what ‘dogmatic common sense’ is supposed to be, especially if it is being attributed to Kant. You offer that ‘Empiricists side with common sense and say that the empirical world is real, in the sense of existing whether perceived or not..’ I would categorize Kant as an Empricist, by this definition, though I think you would not. But what is ‘dogmatic common sense’?
I’m puzzling a bit over whether/how common sense is even capable of grasping the ‘sense’, of dogma? The best that I can do w/this, is that you think Kant’s reliance on sophisticated technical instruments of predication results in garbage, but thankfully, he didn’t entirely give in to this temptation or predilection, though he didn’t have a way of justifying to himself that he didn’t entirely give in. That is, we’re used to the idea of fighting dogma with common sense, but Kant had, strictly speaking, no common sense, because he did not consciously value common sense. He only had, then, ‘dogmatic common sense’.
If Kant finished on the slippery slope to solipsism, then I suppose that you can take a few short solipsistic steps further from some notion in Kant. And if the notion in question is something like ‘‘one is the world, the universe’, then I would understand what you mean. In this case, that would be because you might then take the view, or produce the theory, that only the self really exists, or can be known.
However, as I understand Kant, he would say that if a statement of the nature of absolute reality was ever made, it would be almost always be the product of what we listen to or observe in the physical, external world. I’ll add that I’m of course not completely innocent of interpretations of Kant, that have him saying something like that we (materialists) cannot trust the physical universe to give us the absolute truth. I’ve even encountered the interpretation that Kant had serious solipsistic sympathies. He has interlocutors who accused him of this, in so many words, during his life, by those who categorized him as taking something like idealism on its merits or whatever else and arguing from there, while meanwhile idealism may be characterised as solipsism or ´on the slippery slope towards solipsism´ and such like. I don’t know if you are familiar with Kant’s quote-unquote ‘refutation of idealism’.
Obviously, I bother because I think, at least, that Kant was broadly onto something..what I want to accomplish, is to let you know that while his ideas have been challenged strongly over the years and are unlikely to be true down to every detail, I am not satisfied that you have explored the Kantian alternative for what it really is (in case this might bother you).