The History of Modern Thought in Four Brief Bios—A sidebar to "The Lights by Which We See"
1. John Locke
John Locke was a British philosopher and political theorist whose political thought included the concepts of consent of the governed, natural rights, separation of powers, and checks and balances. All these concepts shaped liberal democracy and are reflected in the U.S. Constitution and Declaration of Independence. As a philosopher, Locke was a leading figure in empiricism, the philosophical position that all knowledge derives from sense perception. According to Locke, the mind is a tabula rasa at birth, a blank slate that receives sensory experiences which are imprinted on it and processed through rational association. We can know nothing by the powers of reason alone or by revelation from on high, said Locke; all we can know is what we experience by direct observation and reflection.
2. George Berkeley
Like Locke, Irish clergyman and philosopher George Berkeley was an empiricist, but he rejected Locke's version as dangerously irreligious. Whereas Locke believed that physical objects exist and that there is a distinction between their material properties (composition, shape, hardness) and our sensations (color, taste, smell) of them, Berkeley argued that the material world only exists in our perceptions of it. In contrast to Rene Descartes' famous philosophical statement, "I think, therefore I am," Berkeley's philosophy could be summed up as, "I am perceived, therefore I am." To critics charging that his philosophy would require that material things disappear when not perceived by humans, Berkeley responded that they are perceived by God, who ensures their continued existence.
3. David Hume
Scottish Enlightenment philosopher David Hume took the empiricism of Locke and Berkeley down the path of radical skepticism. Hume argued that we do not have knowledge in any true sense; we only have ideas and beliefs derived from sense perceptions that we feel are true. For Hume, the principle of cause and effect, one of the building blocks of knowledge, has no objective basis but is simply the result of the mind associating sensory impressions with each other in ways that conform to custom and habit. For instance, Hume would say that when the cock crows at sunrise, we cannot know for sure whether the sun causes the cock to crow or whether the cock's crow causes the sun to rise. Hume's skepticism took a wrecking ball to established views about perception and the value of reason and revelation in the apprehension of the true nature of things.
4. Immanuel Kant
Awakened by the crash of Hume's wrecking ball, the German philosopher Immanuel Kant sought to establish a rational basis for knowledge by synthesizing empiricism and rationalism. Kant agreed with the empiricists that knowledge stems from sensory experience, but he argued that such experience is processed by our rational faculties and is organized according to categories of knowledge (like causes and effects) that are already built into the mind. Kant did agree that our knowledge of things is limited to what is materially perceptible to us. Regarding things inaccessible to the senses (e.g., the self, the soul, God, morality) knowledge is tentative and uncertain.
Kant had the best of intentions: to rescue philosophy from the radical skepticism of David Hume. But by dividing the universe into its sensible and insensible parts, he created a Knowledge/Belief (or Fact/Faith) split that restricted "knowledge" to sensations of the material world, with everything else attributed to "beliefs" shaped by personal experiences and cultural influences. This paradigm shift in thought, called Kant's Copernican Turn, in time led to the denial of moral absolutes and to the popularization of relativism, pragmatism, and utilitarianism.