In the "Critique of Practical Reason" (1787), Kant establishes his most well-known principle, the categorical imperative: "Act only according to that maxim whereby you can, at the same time, will that it should become a universal law." According to Kant, the awareness of this fundamental law of practical reason is a fact of pure reason that no one can escape, and in this fact, he sees the - non-derivable and needing no further derivation - proof of the reality of our freedom, which the "Critique of Pure Reason" could demand in the realm of theoretical reason but could not provide. Alongside the writings of David Hume, who sought an empirical foundation for universally valid ethical principles, Kant's approach of a purely rationally grounded moral law that is binding for everyone remains significant to this day.