This is the second expanded English edition of Kant’s monumental work of philosophy that transformed Western thought – there is philosophy before this book and philosophy after this book.

From wikipedia:

The Critique of Pure Reason (German: Kritik der reinen Vernunft) by Immanuel Kant, first published in 1781, second edition 1787, is one of the most influential works in the history of philosophy. Also referred to as Kant’s “first critique,” it was followed in 1788 by the Critique of Practical Reason and in 1790 by the Critique of Judgment. In the preface to the first edition Kant explains what he means by a critique of pure reason: “I do not mean by this a critique of books and systems, but of the faculty of reason in general, in respect of all knowledge after which it may strive independently of all experience.”

Kant’s influence on Western thought has been profound. Over and above his influence on specific thinkers, Kant changed the framework within which philosophical inquiry has been carried out. He accomplished a paradigm shift: very little philosophy is now carried out in the style of pre-Kantian philosophy. This shift consists in several closely related innovations that have become axiomatic, in philosophy itself and in the social sciences and humanities generally:

Kant’s “Copernican revolution”, that placed the role of the human subject or knower at the center of inquiry into our knowledge, such that it is impossible to philosophize about things as they are independently of us or of how they are for us;

His invention of critical philosophy, that is of the notion of being able to discover and systematically explore possible inherent limits to our ability to know through philosophical reasoning
His creation of the concept of “conditions of possibility”, as in his notion of “the conditions of possible experience” – that is that things, knowledge, and forms of consciousness rest on prior conditions that make them possible, so that, to understand or to know them, we must first understand these conditions
His theory that objective experience is actively constituted or constructed by the functioning of the human mind
His notion of moral autonomy as central to humanity
His assertion of the principle that human beings should be treated as ends rather than as means

Some or all of these Kantian ideas can be seen in schools of thought as different from one another as German Idealism, Marxism, positivism, phenomenology, existentialism, critical theory, linguistic philosophy, structuralism, post-structuralism, and deconstructionism.