From AnthroWiki

The thing-in-itself is, according to the doctrine of transcendental idealism advocated by Immanuel Kant (1724-1804), the absolute transcendental being that exists for itself, which, beyond and independent of any possibility of experience, underlies as actual reality the world of appearances (phenomena) that is the only one that can be experienced by human beings. According to Kant, reality is accessible to us only through the forms of perception of space and time and through thinking in categories, which, however, exist only in the relation of reality to the experiencing consciousness, but are not constitutive of being in itself. The true essence of reality, which lies beyond sensual-categorical experience, is therefore fundamentally inaccessible to man. With this, Kant not only denied the reality of secondary sensory qualities, i.e. qualia such as colour, sound, smell, etc., as John Locke had done before him, but also denied the reality content of primary sensory qualities, i.e. all spatial and temporal phenomena.

„That one can say of a number of their predicates, irrespective of the real existence of external things, that they do not belong to these things in themselves, but only to their appearances and have no existence of their own apart from our imagination, is something that was generally accepted and conceded long before Locke's time, but most of all after his. Warmth, colour, taste, etc., belong to this category. But the fact that, in addition to these, for important reasons, the other qualities of bodies, which are called primarias, extension, place, and space in general, with all that is dependent on it (impenetrability or materiality, shape, etc.), are also considered by me to be mere appearances, against this one cannot adduce the least reason of inadmissibility; and just as little as he who does not want to allow colours to be regarded as properties attached to the object in itself, but only to the sense of seeing as modifications, can therefore be called an idealist: so little can my doctrinal concept be called idealistic, merely because I find that still more, indeed all the qualities that constitute the sight of a body, belong merely to its appearance; for the existence of the thing that appears is not thereby annulled, as in real idealism, but only shown that we cannot at all recognise it, as it is in itself, through the senses.“

Immanuel Kant: Critique of Pure Reason, 1st edition, 1781, AA IV, p. 289

Rudolf Steiner decisively contradicted Kant's view in his fundamental epistemological writings. The fact that the "in itself" of things, their essence, cannot be grasped through sense perception, but can be grasped through thinking in human consciousness, is the foundation of the anthroposophical spiritual science he later founded.

„Anyone who is still looking for something behind things that is supposed to signify their actual essence has not made himself aware that all questions about the essence of things arise only from a human need: to penetrate what one perceives with thought. Things speak to us, and our inner being speaks when we observe things. These two languages come from the same primordial being, and man is called to bring about their mutual understanding. This is what is called knowledge. And this and nothing else is what he seeks who understands the needs of human nature. Whoever does not attain to this understanding, the things of the outer world remain strange to him. He does not hear the essence of things speaking to him from within. That is why he assumes that this essence is hidden behind things. He believes in an outer world still behind the world of perception. But things are only external things as long as one merely observes them. When one thinks about them, they cease to be apart from us. One merges with their inner being. For the human being, the contrast between objective outer perception and subjective inner world of thought exists only as long as he does not recognise that these worlds belong together. The human inner world is the inner world of nature.“ (Lit.:GA 1, p. 333)

„The spiritual content of an external thing that comes to me in my inner being is not something added to the external perception. It is no more so than the spirit of another person. I perceive this spiritual content through the inner sense just as I perceive the physical content through the outer senses. And what I call my inner life in the above sense is not at all, in the higher sense, my spirit. This inner life is only the result of purely sensual processes, belongs to me only as a completely individual personality, which is nothing but the result of its physical organisation. When I transfer this inner life to outer things, I am really thinking into the blue. My personal soul life, my thoughts, memories and feelings are within me because I am a natural being organised in such and such a way, with a very specific sensory apparatus, with a very specific nervous system. I am not allowed to transfer this human soul of mine to things. I would only be allowed to do so if I found a similarly organised nervous system somewhere. But my individual soul is not the highest spiritual thing in me. This highest spiritual must first be awakened in me through the inner sense. And this awakened spiritual in me is at the same time one and the same with the spiritual in all things. Before this spiritual the plant appears directly in its own spirituality. I do not need to give it a spirituality similar to my own. For this world view, all talk about the unknown "thing in itself" loses all meaning. For it is precisely the "thing in itself" that reveals itself to the inner sense. All talk about the unknown "thing in itself" only stems from the fact that those who talk in this way are unable to recognise the "things in themselves" in the spiritual contents of their inner being. They believe to recognise in their inner being insubstantial shadows and schemes, "mere concepts and ideas" of things. But since they do have an inkling of the "thing in itself", they believe that this "thing in itself" is hidden and that there are limits to the human faculty of knowledge. It cannot be proved to those who are caught up in this belief that they must grasp the "thing in itself" within themselves, for they would never acknowledge this "thing in itself" if it were shown to them. But it is this recognition that is at issue.“ (Lit.:GA 7, p. 44ff)


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References to the work of Rudolf Steiner follow Rudolf Steiner's Collected Works (CW or GA), Rudolf Steiner Verlag, Dornach/Switzerland, unless otherwise stated.
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Index to the Complete Works of Rudolf Steiner - Aelzina Books
A complete list by Volume Number and a full list of known English translations you may also find at Rudolf Steiner's Collected Works
Rudolf Steiner Archive - The largest online collection of Rudolf Steiner's books, lectures and articles in English (by Steiner Online Library).
Rudolf Steiner Audio - Recorded and Read by Dale Brunsvold - Anthroposophic Press Inc. (USA)
Rudolf Steiner Handbook - Christian Karl's proven standard work for orientation in Rudolf Steiner's Collected Works for free download as PDF.