Immanuel Kant writes his Critique of Pure Reason in 1781.

Considered one of the most influential works in the history of philosophy, the Critique is an investigation into the foundations and limits of human knowledge, and the extent to which the human mind is able to engage in metaphysics.

Kant builds on the work of empiricist philosophers such as John Locke and David Hume, as well as rationalists such as Gottfried Wilhelm Leibniz and Christian Wolff.

He expounds new ideas on the nature of space and time, and claims to provide solutions to Hume's skepticism regarding human knowledge of the relation of cause and effect, and René Descartes' skepticism regarding knowledge of the external world.

Kant claims to enact a 'Copernican revolution' in philosophy with his doctrine of transcendental idealism, according to which our knowledge does not "conform to objects", but rather objects "conform to our knowledge".

Knowledge independent of experience Kant calls "a priori" knowledge, while knowledge obtained through experience is termed "a posteriori".

According to Kant, a proposition is a priori if it is necessary and universal.

A proposition is necessary if it could not possibly be false, and so cannot be denied without contradiction.

A proposition is universal if it is true in all cases, and so does not admit of any exceptions. Knowledge gained a posteriori through the senses, Kant argues, never imparts absolute necessity and universality, because it is always possible that we might encounter an exception.

According to Kant's doctrine, the human mind shapes and structures the world of experience, making knowledge possible.

Kant claims to have discovered another attribute of propositions: the distinction between "analytic" and "synthetic" judgments.

According to Kant, a proposition is analytic if the content of the predicate-concept of the proposition is already contained within the subject-concept of that proposition.

For example, Kant considers the proposition "All bodies are extended" analytic, since the predicate-concept ('extended') is already contained within—or "thought in"—the subject-concept of the sentence ('body').

The distinctive character of analytic judgements was therefore that they can be known to be true simply by an analysis of the concepts contained in them; they are true by definition.

In synthetic propositions, on the other hand, the predicate-concept is not already contained within the subject-concept.

For example, Kant considers the proposition "All bodies are heavy" synthetic, since the concept 'body' does not already contain within it the concept 'weight'.

Synthetic judgments therefore add something to a concept, whereas analytic judgments only explain what is already contained in the concept.

Prior to Kant, it was thought that all a priori knowledge must be analytic.

Kant, however, argues that our knowledge of mathematics, of the first principles of natural science, and of metaphysics, is both a priori and synthetic.

The peculiar nature of this knowledge, Kant argues, cries out for explanation.

The central problem of the Critique is therefore to answer the question: "How are synthetic a priori judgements possible?"

It is a "matter of life and death" to metaphysics and to human reason, Kant argues, that the grounds of this kind of knowledge be explained.