The Critique of Pure Reason (Kritik der reinen Vernunft), first published in 1781 with a second edition in 1787, is generally regarded as the most influential and widely read work of the German philosopher Immanuel Kant and one of the most influential and important in the history of Western philosophy. It is often referred to as Kant's "first critique," and was followed by the Critique of Practical Reason and the Critique of Judgement.

Kant saw the first critique as an attempt to bridge the gap between rationalism and empiricism — and, in particular, to counter the empiricism of David Hume — famously arguing that, although all knowledge begins with experience, it does not follow that it all arises out of experience.

The Critique is one of the most important works in Western philosophy. In The World as Will and Representation Schopenhauer wrote: "Kant's teaching produces a fundamental change in every mind that has grasped it." It is also notoriously difficult to read and understand. Professional philosopher A.J. Ayer once claimed to not fully grasp Kant's work until having a sunstroke.

The Critique of Pure Reason (CPR) is an attempt to answer two questions: "What do we know?" and "How do we know it?".

Kant approaches the questions by looking at the relationship between knowledge based on reason (what we know purely logically, prior to or independently of experience, or a priori) and knowledge based on experience (what we know based on the input of our senses or a posteriori).

In Kant's view, a priori intuitions and concepts provide us with some a priori knowledge, which also provides the framework for our a posteriori knowledge. For example, Kant argues that space and time are not part of what we might regard as objective reality, but are part of the apparatus of perception, and causality is a conceptual organizing principle that we impose upon nature.

In other words, space and time are a form of seeing and causality is a form of knowing. Both space and time and our conceptual principles and processes pre-structure our experience.

When we see a box as three-dimensional, the shape of the box may not be part of the box's nature. Kant argues that the spatio-temporal aspect of our perception of the shape of the box comes from us, in interaction with the box, not just from the box itself. When we experience events as causing other events, it is because we have a concept of causality in nature into which we fit our experience.

Things as they are "in themselves" are unknowable. For something to become an object of knowledge, it must be experienced, and experience is prestructured by the activity of our own minds -- both space and time as the forms of our intuition or perception, and the unifying, structuring activity of our concepts. These two aspects of our minds turn things-in-themselves into the world of our experience. We are never passive observers or knowers.

Kant's I—the Transcendental Unity of Apperception—is similarly unknowable. I am aware that there is an "I", subject, or self that accompanies all of my experience and consciousness. But since I only experience it in time, which is a "subjective" form of perception, I can never know directly that "I" that is appearing in time as it might be "in itself", outside of time. Thus we can never truly know ourselves as we might be outside of or prior to the forms through which we perceive and conceive ourselves.

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