Key Ideas
How are synthetic a priori judgments possible?
Space and time are forms of our intuition, not features of things in themselves.
Twelve categories (including causality) structure every experience.
The “I think” must be able to accompany all my representations.
The antinomies — reason proves both that the world has a beginning in time and that it does not.
The ontological, cosmological and physico-theological proofs of God all fail.
A famously difficult book: Kant told a friend he had written it in four or five months, “as if in flight”. Its first readers misunderstood it as Berkeleyan idealism, which provoked the shorter Prolegomena to Any Future Metaphysics (1783) and a revised second edition (1787). Scholars cite it by the pages of both: “A” for 1781, “B” for 1787.
The four antinomies
| Thesis | Antithesis |
|---|---|
| The world has a beginning in time and limits in space. | The world is infinite in time and space. |
| Everything is made of simple parts. | Nothing is simple. |
| There is freedom. | Everything happens by natural law. |
| A necessary being exists. | No necessary being exists. |
Kant’s diagnosis: both sides err by treating appearances as things in themselves. For the third antinomy he finds room for both — natural causality for phenomena, freedom for the noumenal self.
Lines to Remember
2 quotationsI had to deny knowledge in order to make room for faith.
Ich musste also das Wissen aufheben, um zum Glauben Platz zu bekommen.
Context & when to use it
By limiting knowledge to appearances, Kant leaves God, freedom and immortality beyond disproof — matters for practical reason and faith, not theory.
When discussing whether reason can prove or disprove God.
Thoughts without content are empty, intuitions without concepts are blind.
Gedanken ohne Inhalt sind leer, Anschauungen ohne Begriffe sind blind.
Context & when to use it
The whole Kantian synthesis in eight words — rationalism without senses is empty; empiricism without concepts is blind.
The perfect one-liner for “theory vs. experience” arguments.
Explore Next
related by shared connectionsProlegomena to Any Future Metaphysics
Kant’s short, clearer restatement of the first Critique, where he credits Hume with waking him from his dogmatic slumber.
Synthetic a priori
Judgments that are informative about the world yet knowable independently of experience. Kant’s master question was how they are possible.
Friedrich Heinrich Jacobi
German critic of the Enlightenment who sparked the Pantheism Controversy and gave philosophy the word “nihilism”.
Critique of Practical Reason
Kant’s second Critique — the moral law is a fact of reason, and through it we know we are free.
Scholasticism
The method and philosophy of the medieval universities — rigorous disputation, the reconciliation of faith and reason, and the absorption of Aristotle into Christian, Jewish and Islamic thought.
Continental Rationalism
The seventeenth-century conviction that reason alone, unaided by the senses, can reach certain knowledge of the deepest truths.