Critique of Pure Reason (book, Immanuel Kant)

Introduction:

Metaphysics uses reason to try to answer the most difficult questions, but reason has continually failed. People are now using this fact as an excuse to ignore the difficult questions of metaphysics and discourage people from asking them. But, it's in our nature to ask these questions.

So, I've solved all the metaphysical problems by applying rigorous reasoning to reason itself. I do not use opinions in this book. I rely only on axiomatic truths and the principles which rigorous logic can build up from those axioms. I trust the reader will question me at every turn and find that I'm telling the truth.

The Transcendental Aesthetic

On Space

Space can't be a concept that we've learned simply about by observing the world, because we can't even conceive of a world beyond ourselves without the concept of space. Therefore, space is an a priori concept, something that has always been present in our minds. It's a pure intuition, present in us prior to any observation. The fact that we conceive of space as infinite, despite infinity being beyond our understanding, further indicates this.

If we want to build metaphysics up from the understanding that space is an a priori concept, we need to make sure that the concepts we intuit actually can be intuited directly from the concept of space, and that these new concepts can only arise given the conditions created by the a priori concept of space.

Thus, space cannot be understood as a property of any things in themselves or in their relation to one other. Space is instead just the way we see the world. We can only speak of space from the human standpoint.

The same, however, cannot be said of things like color, taste, sight, hearing, etc., since these concepts are experienced directly through experience and cannot be separated from their sensations.

On Time

Like space, time can't be a concept that we discover by observing the world, since the idea of things happening at the same time or successively doesn't make sense without the already present concept of time. Therefore, time is another  a priori  concept.

Just as different spaces cannot be successive but are always simultaneous, different times cannot be simultaneous but are always successive.

From time and space, we can intuit the concepts of alteration and motion (alteration of place).

Space is limited as an a priori condition to the outer world. But time is an a priori condition of all experience. Time is the means by which we experience experience. It is the form of our inner sense.

Like space, however, time is merely a subjective condition. Beyond human subjectivity, time is nothing and has no absolute reality.

The Transcendental Logic

On logic in general

The things we think come from two sources: our immediate impressions and the mental tools we use to make sense of those impressions and turn them into thoughts. When thoughts involve a physical object, they are empirical. When not, the thoughts are "pure." The former we might call sensibility and the latter understanding.

Aside from being either pure or empirical (aka applied), logic can also be general or particular, and we can create four combinations from these two sets of categories. For the general logics: I should note here that not even a priori thought is truly "transcendental." Rather, the means by which we think through a priori and empirical thoughts is transcendental.
 * A general pure logic deals strictly with a priori principles--no reference to the objective world. This is the only pure science, but there's also not much to work through and it's a bit dry.
 * A general applied logic deals with the rules for how we can logically think through empirical situations.

What is truth? How do we know which thoughts are real and which are false? We can't exactly. General logic supplies us with the rules for how we might think through the objective world, but it doesn't supply the truth of that world. General logic is in this sense just a "canon" for thought, not an "organon" that produces truth.

Transcendental Logic: First Division