The narrator refers almost casually to Immanuel Kant (1724-1804), a towering 18th-century German philosopher who wrote about the theory of knowledge in a way that continues to be influential even into the 21st century. His Critique of Pure Reason
attempted to find a sure way of observing cause and effect without having to depend solely on the human senses. He accomplished this (or believed he had) by positing a priori truths: statements such as those found in geometry or Newtonian physics that are true beyond any need for empirical observation.
“In Kant’s language,” the narrator of Unbearable Lightness remarks, “even ‘Good Morning,’ suitably pronounced, can take the shape of a metaphysical thesis.” The narrator then shows that the passage of time converted a minor, almost joking scrap of conversation that inspired a Beethoven composition (“Es muss sein!”) into something weighty and fateful.