We think of Hegel not only as one of the most important philosophers but also as one of the most important representatives of the philosophy of identity. I understand here by ‘philosophy of identity’ that type of philosophy that states the fundamental identity or correspondence between the human mind and external reality. Because of its support for this principle, Hegel’s philosophy is considered an idealism, particularly an absolute idealism. In this respect, we must add a few remarks concerning idealism vs realism.
The distinction idealism vs realism or empiricism is a modern distinction. We will not trace here all the history of this distinction. It is enough to say that idealism is the philosophical point of view that considers reality a product of the human mind, whereas realism considers knowledge and the human mind products of nature or external reality.
The modern idealist view that interpreted the human mind as creating reality was possible due to Descartes’s distinction between the extended or material substance and the cognitive substance. This distinction made it impossible to think of the correspondence between these two substances as something natural, because they could not act upon each other.
We will leave aside the idea of the pineal gland, and emphasize the fact that Descartes needed the idea of God as the warranter of the truth of our knowledge, understood as correspondence between the content existing in our mind, or within the cognitive substance, and the outer reality, or the content of the extended or material substance.
The idea of the cognitive substance brought about the representation that we are, so to speak, enclosed inside our minds, without any means to overcome this isolation. Berkley followed this path of thought, developing his subjective idealism, and Leibniz, in the same vein, theorized the idea of the monad, which has no windows to establish a continuity with the external world.
Kant took over this paradigm of human subjectivity entirely separated from the external world. But, unlike his predecessors, he no longer based the coherence of our knowledge on God’s power but on internal rules and criteria.
Unlike his predecessors, Kant thought of human reason and knowledge as a system. As such a system, reason could not have any foreign elements or heteronomy. Human reason and knowledge had to be seen as structures grounded on logical necessity, which could not be maintained if any of their elements or rules were simply taken over from outside. In that case, we would have facts but no necessity. This is the most important feature of the new content of the distinction idealism vs realism in Kant’s philosophy.
Why had human reason to be thought of as a system? It was necessary because reason was the object of transcendental philosophy, which was a part of metaphysics. And Kant hoped to transform this metaphysics into science, since science was, for him, the true form of knowledge.
He considered that mathematics, pure physics, and logic managed to become sciences. What characterized these disciplines as sciences? In Kant’s view, it was their ‘mathematical’ or ‘geometrical’ method, which, of course, consisted not in dealing with numbers but in demonstration.
The demonstration involved necessity of thought. Therefore, whatever type of knowledge had not yet reached the stage of describing its object not rhapsodically but using necessary logical deductions could not be called a science. Of course, Newton’s physics was the most relevant science in this respect.
The quality of necessity concerning propositions of sciences cannot be acquired otherwise than through deduction. However, with respect to the principles of science, the necessity of thought implies evidence by itself. Therefore, a true science, in Kant’s sense, implies deductive knowledge based on principles.
Thus, if we were to conceive of metaphysics as science, we necessarily have to conceive of reality – the object of this science – as being a system. Of course, neither Kant nor anyone else could say that reality is in itself a system. But one could say that our knowledge about reality is a system – which indeed happened with mathematics and Newtonian physics.
But how is it possible to have systematic knowledge about reality without thinking of reality as being also systematic? Stating something about the whole of reality in itself would have meant having access to infinite knowledge, to be a God. And this, Kant rejects, obviously.
Then, the next step is to maintain that reality is given to us through a filter that shows it to us as a system. And this is precisely what Kant did: to make metaphysics possible as science, he conceived of reality as a systematic phenomenal world.
Thus, in Kant, the principle of identity (and consequently also the distinction idealism vs realism) takes a new shape: it is the identity between thought and the phenomenal world. This is why Kant calls his philosophy transcendental idealism. This is, in other words, a type of idealism that considers not that the world itself is entirely only in our mind but that the world’s phenomena are entirely only in our mind.
And, as a spectator of phenomena, the Kantian subject is almost a Leibnizian monad that has no connection with the world in itself. It is true, there is the rest of the so-called ‘thing in itself.’ Still, since this concept is rather only a limit of our knowledge, we cannot be sure of its meaning.
In other words, is it something external to us that we must posit or is it the expression of our impossibility of overcoming phenomenality? And in this respect, something that we only imagine to exist but that might not exist?
Now, we must concede that Kant probably regarded the first meaning as being the true meaning of the concept of the thing in itself. However, his contemporary readers found great difficulties in accepting such a meaning.
It was not only because the idea of an external thing in itself affecting the human senses contradicted Kant’s requirement to limit the use of the categories – in this case, of the category of causality – only to the world of phenomena. That concept also ruined the representation of human reason rooted in its own highest principles.
The thing in itself was a foreign piece in Kant’s architectonic of reason. Jacobi expressed the puzzle concerning the thing in itself that confronted those readers as follows: without the concept of the thing in itself, one cannot enter the system of the Critique of Pure Reason. But with that concept, one cannot remain within that system.