Modern philosophy: Descartes, Locke, Hume, Kant

How do you know the Sun will rise tomorrow? “It always has” — but is that a proof? In the 17th and 18th centuries, as modern science was being born, philosophers asked: what can we know for certain, and where does our knowledge come from — from reason or from experience?

Descartes: doubt everything

The French mathematician and philosopher René Descartes (1596–1650) — the man behind “Cartesian coordinates” — searched for a firm foundation of knowledge in his Discourse on the Method (1637) and Meditations on First Philosophy (1641). His method of doubt: set aside everything that can be doubted. The senses sometimes deceive; perhaps I am dreaming; perhaps a cunning “evil demon” deceives me even about mathematics. Yet while I doubt, I am thinking, and if I think, then I exist. This became Descartes's first certain truth.

I think, therefore I am.

On this foundation Descartes rebuilds knowledge: clear and distinct ideas are true, and some ideas — such as God and mathematical truths — are innate. The world consists of two substances: thinking substance (mind, soul) and extended substance (body, matter). This is dualism; the question of how mind and body interact later became a major problem.

Rationalism and empiricism

Rationalists (Descartes, Spinoza, Leibniz) saw reason as the main source of knowledge. Empiricists (Locke, Berkeley, Hume) held that all knowledge comes from experience. In An Essay Concerning Human Understanding (1689), John Locke (1632–1704) wrote that at birth the mind is like white paper, with nothing written on it (in Latin, tabula rasa — “a blank slate”). Simple ideas come from the senses and from reflection and then combine into complex ideas. Locke distinguished primary qualities (size, shape, motion — in the object itself) from secondary qualities (colour, taste, smell — dependent on the perceiver). George Berkeley (1685–1753) went further: “to be is to be perceived”.

Hume: habit instead of certainty

The Scottish philosopher David Hume (1711–1776) took empiricism to its limit in A Treatise of Human Nature (1739–1740) and An Enquiry Concerning Human Understanding (1748). He separated relations of ideas (mathematics: true by definition, but telling us nothing about the world) from matters of fact (known only through experience, and could have been otherwise). One billiard ball strikes another and the second moves. We do not see any “necessary connection” — only that one event is always followed by the other. Our belief in causation comes from habit.

This leads to the problem of induction: we assume the future will resemble the past, but we cannot prove it — saying “it has always been so” is itself an induction, so the argument is circular. Hume also showed that no conclusion about what ought to be follows logically from statements only about what is.

Kant: a “Copernican revolution” in philosophy

Immanuel Kant (1724–1804) of Königsberg wrote that Hume had woken him from his “dogmatic slumber”. In the Critique of Pure Reason (1781) he tried to unite the two camps: all our knowledge begins with experience, but not all of it arises from experience. The mind actively organises experience: space and time are the forms of intuition, and categories such as causality are the forms of thought. This is Kant's “Copernican revolution”: our knowledge does not conform to objects; rather, objects as they appear to us conform to the structure of our cognition. We know phenomena (things as they appear), while “things in themselves” (noumena) remain unknowable.

Thoughts without content are empty, intuitions without concepts are blind.

1. 1637
   Descartes, Discourse on the Method
2. 1641
   Descartes, Meditations on First Philosophy
3. 1689
   Locke, An Essay Concerning Human Understanding
4. 1710
   Berkeley, A Treatise Concerning the Principles of Human Knowledge
5. 1739–1740
   Hume, A Treatise of Human Nature
6. 1748
   Hume, An Enquiry Concerning Human Understanding
7. 1781
   Kant, Critique of Pure Reason
8. 1784
   Kant, “What is Enlightenment?”