(De. Kritik der reinen Vernunft). First edition (A), 1781; second edition (B), 1787.

Immanuel Kant's seminal work on the possiblity of metaphysical knowledge. The work supersedes Kant's inaugural dissertation, expounding what is now known as his critical philosophy.

The Critique concerns itself with the possibilty of a priori synthetic cognition (Erkenntnis). Cognition (often translated as `knowledge') is a priori when it precedes experience; that is, when it is completely non-empirical in origin. Cognition is synthetic when it seeks to relate two different things; the opposite is `analytic' cognition, which occurs when the subject is `contained in' the predicate (that is, analytic cognition produces nothing new). Kant argues that synthetic a priori cognition is necessary for a science of metaphysics.

In addition to defining the conditions for metaphysical knowledge, Kant places bounds on our metaphysical knowledge. That is, he demonstrates certain things that are fundamentally unknowable through reason. For example, we can never know things in themselves---we only have experience of appearances, and cannot extend our knowledge beyond.

Among other things, Kant proves in the Critique that:

The work is divided into a hierarchical structure:

  1. Preface
  2. Introduction
  3. Transcendental Doctrine of Elements
    1. Transcendental Aesthetic
    2. Transcendental Logic
      1. Transcendental Analytic
      2. Transcendental Dialectic
  4. Transcendental Doctrine of Method

The standard English translation is that of Norman Kemp Smith, though there is a more recent, quite good, one by Paul Guyer and Allen W. Wood (Cambridge Press).

Kant's Prolegomena to Any Future Metaphysics is a partial guide to the Critique. The Critique itself paves the way for Kant's later works, such as Groundwork of the Metaphysics of Morals, Critique of Practical Reason, Critique of Judgement, and Religion Within the Bounds of Reason Alone.

Immanuel Kant was born, raised, and educated in the middle of the 18th century. This period, the heart of the Enlightenment, had already given rise to great works of science and philosophy that are still part of the foundation of modern learning. Kant received his higher education at the University of Königsberg in East Prussia, where he was later appointed professor of philosophy. After thorough study of the works of Descartes, Leibniz, Spinoza, and others, Kant’s study of philosophy and metaphysics eventually led him to ask one question: "How do we know that metaphysics, as a pure science, is valid?" This question, and his search for the ability to do logically valid philosophy, led Kant to publish one of his two most famous works, the Critique of Pure Reason. In this volume Kant deals with questions about how science is possible, what makes mathematics work, and whether the same reasoning can be applied successfully to philosophy, specifically metaphysics. His answers to these questions laid a new groundwork for philosophy and give startling insights into how we perceive the world around us and how nature works.

Kant introduces his seminal work by differentiating between what he calls the two styles of truth. Kant shows that there are two types of truth: the analytic and the synthetic. An analytic truth, says Kant, is one that merely elucidates or expounds the subject of it. In other words, the predicate is a part of the subject. For example, to say "all bachelors are unmarried" is simply to explain the inherent meaning of bachelor. If a man were married he could not be a bachelor. This is an analytic truth. A synthetic truth, in contrast, extends our knowledge of a subject. To say, for example, that "all bachelors are less than 15 feet tall" would give us more knowledge about bachelors than we previously had. It is not inherent in the definition of a bachelor that they must be less than 15 feet tall, but it is still certainly true.

From this discussion Kant proceeds to review a well-known philosophical concept: that of truth which is knowable a priori, or by the mind alone, and that which are only knowable a posteriori, or with the aid of experience or sensation. He concludes that all analytic truth is knowable a priori, because it can be analyzed by the mind alone. If a definition is known then all analytic truth can be derived from it without the aid of experience. Similarly, it seems that all synthetic truth is necessarily only knowable a posteriori. Without experience, how is it possible to know anything that not already contained in the definition of the subject? Kant’s answer to this question, and the foundation of his Critique, is reached by a foray into that most beautiful of the sciences, mathematics.

Kant begins with the question, "Is it possible to know synthetic truths a priori?" Since all truths in mathematics are necessarily known a priori (by the nature of mathematics), Kant begins his search there. An analysis of simple arithmetic yields very promising results. Examining a simple arithmetic equation (e.g. 2 + 3 = 5) shows that such a truth is indeed synthetic. The concept of "five" is nowhere contained within the definition of "two", nor "three". Similarly the definition of addition and equality nowhere contain the concept of this number "five". How then can we know that 2 + 3 = 5? Kant’s answer lays the groundwork for the rest of his treatise. Because of what "two" and "three" represent, the way we comprehend quantities in general, two plus three must equal five, for we cannot comprehend the world in any other manner. Kant thereby concludes that this quality, the ability to know synthetic truths a priori, is the necessary trait for any true science. For a science to be valid it must be able to find certain synthetic truths without the aid of experience. In more colloquial language, the mind must be capable of thinking true things.

Kant answers the question of "How is pure mathematics possible?" with his simple example of 7 + 5 = 12. He shows that mathematics, through logic applied to a priori axioms, can derive other synthetic truths without the aid of empirical observation. This is the foundation of all possible sciences and so he demonstrates handily that mathematics must indeed be possible. A similar style applies to answering his second question, "How is pure natural science possible?" A brief review of the concepts contained in natural philosophy, or physics, reveals that they too can be used to derive other synthetic a priori truths, which might be verified by experience, but are true regardless, and expound a new idea not contained in the original concepts. As with mathematics, this makes pure natural science possible.

Kant then turns to the question of "How is metaphysics as a natural predisposition possible?" Kant realizes that since the beginning of recorded human history, men have had the ability to think, and the inclination to use reason to determine fundamental truths about the universe. (This is the science first called metaphysics by the students of Aristotle.) Also, however Kant is critical of the success of this "science" thus far. He notes that not only has little progress been made in the field since its ancient inception, but also every study of it has led to irresolvable contradictions, which should certainly not be for a science of any sort. This leads him to conclude that before metaphysics is undertaken, man must first examine his own processes of logic, and determine what can (or cannot) be known through pure reason.

This leads him to his last question, "How is metaphysics as a science possible?" As we saw previously, Kant relies on the discovery of synthetic a priori truths to validate a science. Kant believes that there are several of these that apply to metaphysics. These include, "All effects have causes," and "God exists." However, Kant is interested in more than simply its possibility. As stated previously, he wants to set the bounds of where logic can and cannot go. What is knowable? What is unknowable? These are his questions, and he probes the metaphysical relationship between logic and reality with an investigation of the nature of space and time.

Kant’s logical analysis of the concept of space leads to some interesting conclusions. He reasons that since we cannot understand the concept of space without it having certain properties not inherent to it, we must have come to those conclusions (about space) a priori, and that those conclusions must also be synthetic (as nothing about space necessarily dictates them – they are simply required for us to understand reality.) He asks, "How can we have these synthetic a priori truths about this concept?" The answer, Kant proposes, is that space is a concept that our minds project onto the actual reality we are incapable of perceiving. Space is a form of the minds way-of-knowing, not an underpinning of the true metaphysical reality of things-in-themselves. Kant applies the same logic to the concept of time, and concludes that both space and time are simply methods the mind uses (hard-wired into us, if you will) to process the information that reality sends it.

This conclusion is a startling one, and leads to Kant’s goal of a boundary on what metaphysics can and cannot do or know. He realizes that while it is possible to analyze many things about the nature of the universe, because the mind must project its own concepts onto "things-in-themselves" to comprehend them, that the actual things-in-themselves are very much unknowable. This gives a clear indication of how metaphysics is possible as a science, and what it can and cannot do in the hands of humankind. It is possible and useful for an analysis of the world at hand, but we can never have any true knowledge of things "as they really are." Our pure knowledge is not truly possible – we are warped by our minds methods of knowing things. However, practical knowledge of the world around us is possible and useful.

Kant’s treatment of his questions surrounding the possibility of science and metaphysics leads to very interesting conclusions. Kant systematically explains how truth is obtained synthetically, but a priori, uses that ability as the foundation for true science, and shows that this allows metaphysics to be a science – but only as far as the world itself. Because our ways of knowing project certain truths onto our minds, we cannot actually come to true conclusions about reality as it actually is – only as we can perceive it. Thus while pure reason or cognition is impossible, practical knowledge is certainly not. This treatment of metaphysics revolutionized the field of philosophy – the empiricists were wrong because they did not begin where a scientist should (synthetic a priori truths) and the rationalists were wrong because they did things that they could not do (attempted to know the true nature of things outside our perceptions.) By showing that both of these things were impossible, Kant set the stage (with a mere four questions) for a new era in modern philosophy, and is widely regarded as one of the greatest thinkers of our time.

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