If you have a sensation, i.e., interaction with the external environment, what is its impact on you as a living being? To be able to have a sensation, you must be able to distinguish that stimulus from another stimulus. This cannot be a subjective capacity but must be a feature of the world because if there were no differences in the world, you could not discern one through your sensory organs.
But is it possible to have a red sensation if there were no red things in reality out there? Indeed, in order to discriminate among sensations, the stimuli must already be different, but what is the immediate relationship between a stimulus and its associated sensation?
We would be pretty almighty wizards if we were able to create such an infinity of different qualities – like those we perceive almost every day – starting from a stuff devoid of qualities, don’t you think? And, since there must be a difference between stimuli so that we can interact with them, it means that the endless number of qualities we perceive must exist objectively, at least concerning their differences.
Red and black may not exist objectively as such, but a difference between what corresponds externally to our red and our black must exist; otherwise, we would be the creators of this difference. And then there are innumerable nuances of red or any other color in general. Again, if we do not believe we are almighty wizards, those shades are not creations of our minds but must be given to us.
The Newtonian theory of light teaches us that colors are waves and that the quality of a color does not exist in truth, but it is only that waves of light pervade external space. We are thus bathed in a myriad of different luminous waves.
However, we can only grasp colors with the surfaces on which they are spread. This is why we say that colors belong to things and are their attributes. This means that we are unable to grasp a color if it is not associated with something consistent and is ineffable, like a pure light wave.
The color must be located in a spot and not spread throughout the universe. This is why it is not by accident that common speech tells us that colors belong to things.
Be that as it may, those waves coming from the colored surfaces of things are expressions of some interactions between light waves and the material things on which they are spread, or are direct results of the emanation of those material bodies. The red apple is not dyed red, but its redness emanates from the apple itself. In this regard, the redness is its quality, or, as the ancients said, its way of being. Qualities are ways of being.
We find thus that one of the Kantian categories cannot belong only to the human mind but must have an objective correspondent, the category of substance and inherence, of one thing and its properties. The external stimulus must have a spatially located intensity in its action on us so that we may discern it; that is to say, it must somehow have a confined shape.
That means, also, that space cannot be exclusively a human a priori intuition, but rather is a precondition of Being itself. Things, in general, cannot interact with each other if they cannot locate each other and spot each other’s qualities. Whatever I experience as being different from me must be experienced in a different place than where I am.
Of course, this experience is not necessarily yet the fully articulated spatial experience we humans have. Kant rightly says that we cannot experience something if we do not think of it as a unity that supports all its attributes.
Now, the point is not only that we cannot experience it, but we could not even interact with it if it were not confined. So we need things around us to be material in order to make their experience: things that are unities and endowed with many properties. The Kantian productive imagination does not create the sensation but only what we could call the form of sensation, i.e., its continuity and space attribution, a form based on the a priori intuitions of time and space.
However, the same thing happens here as with the famous problem of causality as a category of the human mind. Kant was criticized even during his lifetime in this respect. It was said that one could never have any experience if it were not attributed to real external things, because we should deny the objective causal action of things in themselves on human knowledge.
And Kant indeed accepted causality as a possible objective feature of things, as far as we call them noumena, that is to say things that are not given in our sensibility. But, in that case, he denied any connection with the intuition of time. This lack of connection stops us in fact from really knowing those things.
In the same way, we should say that spatiality – and with it form, i.e., property, too – must exist beyond the human mind, because otherwise we simply could not interact with things. It is not only that we could not have their experience as knowledge, but that nothing could act on us if it lacked (some sort of) spatiality, i.e., if it were not a somehow spatially confined center of action.
Of course, nothing from all this is thinkable once you fully enter the Kantian game and accept the premise that our sensations are just ‘matter’ of knowledge and, therefore, implicitly, they must have a chaotic character, which awaits the ordering power of human a priori capacities.
Kant seems to have apprehended these possibilities. This may be why he revised his approach in the second edition of his Critique of Pure Reason. Whereas in the first edition, he tried to show how human categories build themselves in our minds, in the second edition, he seems to abandon this track, being interested instead in the critical dimension of his approach, namely in showing how these categories are the necessary elements of our knowledge and why they limit our knowledge to our experience.
Such an interpretation is compatible with his claim that there exists a history of pure reason, namely that humans develop their knowledge, and in this knowledge, their concepts become more and more accurate, aiming at reaching the ‘sure path of science.’ But on the other hand, such a hypothesis also allows continuity with German Idealism, especially with Hegel and his historical understanding of philosophy.