(Note. This was prepared for my students on the spur of the moment many years ago. Over 200 years of philosophy in 2 pages!)

Descartes wants to know what’s true. He begins by doubting everything and argues that knowledge derives from the certainty of the existence of one’s own consciousness and the innate ideas it holds. Primary among these innate ideas are mathematical ideas and the idea of a God. Upon this foundation, he claims all knowledge is built.

Locke argues that innate ideas are just another name for one’s pet ideas. Instead, he argues, knowledge is based on sense-data. Locke realizes that we only know things as we experience them, we don’t know the essence of the substances that make up the world. Retreating from the skepticism this implies, he accepts the common sense view that our perceptions correspond to external substances that are in the world.

Berkeley realizes that we can have perceptions without there being an external world at all. He believes that things exist only to the extent they are perceived, and thus non-perceived things don’t exist. All reality may be in the mind! Recognizing the implications of this radical philosophy, Berkeley claims that his God constantly perceives the world and thus the world is real after all.

Hume follows this thinking to its logical conclusion. We have perceptions, but their source is unknown. That source could be a god or gods, other powerful beings, substances, the imagination, etc. He also applies this skepticism about the existence of the external world to science, morality, and religion. Scientific knowledge is not absolute because there are problems with the idea of cause and effect as well as with inductive reasoning. Still, Hume believes that mathematics and the natural sciences are sources of knowledge.

Hume’s attack on religion is one of the most famous in the history of philosophy, and he ranks with Feuerbach, Marx, Nietzsche, and Russell as great critics of religion. His argument that miracle stories are almost certainly untrue is the most celebrated piece on that subject ever written.

For example, take the case of virgin births or resurrection from the dead. Such stories are found in many religions and throughout pagan mythology. But Hume asks whether it is more likely that such things actually happened, or that these are myths, stories, lies, deceptions, etc. Hume argues that it’s always more likely that reporters of miracles are deceiving you or were themselves deceived than that the supposed miracle actually happened. Lying and being lied to are common, walking on water and rising from the dead—well, not so much.

Hume’s philosophy set the stage for the greatest of the modern philosophers, a man who said that Hume had “awakened him from his dogmatic slumber.” This thinker wants to respond to Hume’s skepticism and show that mathematics, science, ethics, and the Christian religion are all true. His name was Immanuel Kant.

Kant was one of the first philosophers who was a professor. He was a pious Lutheran, a solitary man who never married, and the author of some of the most esoteric works in philosophy. Troubled by Hume’s skepticism, Kant looked at both rationalists like Descartes and empiricists like Locke, Berkeley, and Hume for answers. Kant believed that the problem with rationalism was that it established great systems of logical relationships ungrounded in observations. The problem with empiricism was that it led to the conclusion that all certain knowledge is confined to the senses.

Kant thought that if we accept the scientific worldview, then belief in free will, soul, God, and immortality was impossible. Still, he wanted to reconcile rationalism and empiricism, while at the same time showing that God, free will, knowledge, and ethics are possible. In fact, much of western philosophy since Descartes has tried to reconcile the scientific worldview with traditional notions of free will, meaning, God, and morality.

Kant’s Epistemology – Kant argues that rationalism is partly correct—the mind starts with certain innate structures. These structures impose themselves on the perceptions that come to the mind. In other words, the mind structures impressions, and thus knowledge results from the interaction of mind and the external world. Thus both the mind and sense-data matter in establishing truth, as the success of the scientific method shows.

Kant’s Copernican revolution placed the mind, rather than the external world, at the center of knowledge. What we can know depends upon the validity of what’s known by the structures of the mind. But is metaphysical knowledge justified? Can we know about the ultimate nature of things, things beyond our experience? Can we know if God, the immortal soul, or free will exists?

What he realized was that all we can know are phenomena, that is experiences or sense-data mediated by the mind. Since all our minds are structured similarly, we all generally have the same basic sensory experiences. But we cannot know “things-in-themselves,” that is, things as they actually are. Thus there is a gap between human reality—things as known to the mind—and pure reality—things as they really are.

To bridge this gap Kant proposes regulative ideas—self, cosmos, and God—which serve to make sense of our experiences. We must presuppose a self that experiences, a cosmos to be experienced, and a cause of the cosmos which is God. Kant grants that we can’t know if any of these things are true, but he thinks it is a practical necessity to act as if they are. We cannot have experiences without there being a knower, a known, and God. Since we do have experiences, Kant concludes that these regulative ideas probably correspond to real existing things.

Summary – Descartes was responding to the faith of the Middle Ages. His skepticism led eventually to the full-blown skepticism of Hume. Kant tried to reconcile Cartesian rationalism and Humean empiricism. He tried to harmonize science with religion, reason with ethics, and more. Whether he was successful is another question.

(Here is a two-page summary of Kant’s ethical theory. And here. is the theory in detail.)

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