Summary: Critique of Pure Reason (page 12)

Things are spatio-temporalised, and this is what allows us to have an experience of them. However, in the very process of being shaped by space and time, their true nature—the thing-in-itself—remains forever inaccessible, as it has been irreversibly modified.

Thus, if we were to mentally remove our subjectivity, or even just the subjective constitution of the senses in general, all properties and relations of objects in space and time—as well as space and time themselves—would disappear. As phenomena, they cannot exist in themselves but only in us¹.

With Kant, the thing-in-itself becomes unknowable:

As for what could be considered a characteristic of objects in themselves, independent of our receptivity through sensibility, this remains entirely unknown to us.

[And even if] we were able to bring our intuition to the highest degree of clarity, we would be no closer to the nature of objects in themselves. For in any case, we would know only our mode of intuition—that is, our sensibility, which is subject to the conditions of space and time that originally belong to the subject. Whatever objects may be in themselves, we could never know them, even if our knowledge of phenomena attained the greatest possible clarity, for only phenomena are given to us.

We cannot know things-in-themselves—only phenomena. But it is crucial to understand that this does not mean that what we know is mere illusion. That would be a major misunderstanding.

First of all, the phenomenon is connected to the thing-in-itself. It represents the thing-in-itself—albeit in a modified form—but it still retains a certain relation to it.

Moreover, we can fully understand everything that pertains to the phenomenon. A domain is thus defined—the phenomenal realm—which can be explored and discovered in its truth. We must simply remember that the truths we uncover are phenomenal truths and do not concern the thing-in-itself.

Finally, the fact that our cognitive faculty has a particular form underlies the universal and necessary character of a priori judgements relating to it (such as mathematical judgements). For instance, we are assured of the truth of geometrical judgements, insofar as they pertain to the pure form of intuition, namely space.

We can now better understand certain references in the Preface. Take, for example, the following passage from the Preface to the Second Edition:

Until now, it was assumed that all our knowledge had to conform to objects […]. Let us therefore attempt, just once, to see whether we might not achieve better results in the problems of metaphysics if we assume that objects must conform to our knowledge.

We now understand why Kant says that objects conform to our knowledge: this is because they must conform to the form of our understanding—space and time—in order to be perceived and to become objects of experience.

It would indeed be a miracle if our cognitive faculties were so constituted as to grasp things-in-themselves in their absolute truth.

Furthermore, this is the only way to explain the enigma of how we can possess a priori knowledge (as in mathematics). It is a mystery, after all: how can we know something without first having experienced it? The answer is simple: we possess a priori knowledge because we possess a priori forms of sensibility and understanding.

And what are the a priori forms of understanding? It is now time to move on to the Transcendental Logic.

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¹ Our translation. The references for the quotations are available in the book Kant: A Close Reading