Frederick II’s preference for writing and speaking in French – he once said provocatively (and falsely) that he spoke German only with his horse – increasingly branded him in the eyes of the educated middle class as a man of the past. It was they, too, who formed the greater part of the German reading and theater-going public, and of the intellectually ambitious group that struggled to comprehend the thought of Immanuel Kant.
Kant synthesized many strands of the German and European Enlightenment in a body of ideas still widely regarded today as the greatest philosophical achievement since Aristotle. In three books of the 1770s and 1780s (the critiques of “pure reason” or rational knowledge, “practical reason” or morality, and aesthetic “judgment”), Kant set Enlightenment thought on new foundations. In response to advancing eighteenth-century philosophical skepticism (such as David Hume’s) questioning the inherent rationality of nature (and human freedom too), Kant argued – in his self-described “Copernican revolution in philosophy” – that it was not the realm of things outside human consciousness that was necessarily and ascertainably rational. It was, rather, the human mind itself, which was so structured as to organize all perceptions according to the categories of space, time, and causality. The human mind does not mirror a rational nature, but constructs it.
Human reason, Kant said, is nature’s lawgiver. Nature may indeed be inherently rational. Though the mind cannot know this with certainty, it must try to comprehend the “thing-in-itself” outside human consciousness, such as the physical universe, as if it did possess the attributes reason ascribes to it. As for morality, while reason will argue that all human actions are causally explicable by pre-existing conditions, and in that sense pre-determined, our possession of an unconstrained moral will – an idea Kant shared with the French philosopher Jean-Jacques Rousseau, whom he admired – enables us to act freely as moral agents if we consciously chose to do so. The moral law resides, not outside us, but within us, creating a potential – still far from fully realized – for ethical self-determination independent of divine power. As for aesthetics, the artist’s mission is, similarly, not to bow to external authority, but to generate from within an independent and autonomous creative will.
Kant’s political philosophy, which, on account of Prussian censorship during the period of the French Revolution, was difficult for him to express with full freedom, emphasized the primacy of government under the rule of law (the Rechtsstaat or “state of law”). He insisted on the separation of executive and legislative powers, but left parliamentary or representative government in theoretical shadow. His philosophy of history proposed that advancing trade (the sphere of the commercial and industrial middle classes) would work to unify the states of the world, bringing war into such disrepute – and making it so counter-productive economically – that perpetual peace would result.
This was a vision of emancipatory progress in history typical of the Enlightenment. In his social and economic thought, Kant was an admirer of the Scotsman Adam Smith, another towering figure of the age, who argued for the interplay in free capitalist markets of supply and demand undistorted as far as possible by state power. Altogether, Kant stands as the philosophical godfather of nineteenth-century German liberalism.