Table of Contents

  1. John Locke: From Introspection to Modified Common Sense
  2. Introspection
  3. Scientific Common-sense View of Perception
  4. Locke’s View
  5. Locke’s Arguments
  6. Opening the Door for Berkeley, Hume, and Kant
  7. Addendum: Demonstrative and Probable Reasoning

John Locke:
From Introspection to Modified Common Sense

  • The writings of John Locke “lie at the foundation of modern philosophical empiricism and political liberalism. He was an inspirer of both the European Enlightenment and the Constitution of the United States.” (Britannica)
  • Masterwork
    • An Essay Concerning Human Understanding 1689
    • “Along with the works of Descartes, the Essay constitutes the foundation of modern Western philosophy.” (Britannica)


  • Locke and the other British Empiricists began their inquiries with introspection:
    • “Introspection is the process of observing the operations of one’s own mind with a view to discovering the laws that govern the mind” (Britannica)
  • Locke’s Principles of the Operation of the Mind:
    • All ideas come from experience.
    • An idea is any “object of the understanding when a man thinks.”
    • All ideas are simple or complex.
    • All complex ideas are derived from the mind’s combining simple and complex ideas.
      • The complex idea of a snowball, for example, consists of the simple ideas of whiteness, roundness, solidity, coldness, and so on.
    • All simple ideas come from Sensation or Reflection
      • Sensation includes sense-experiences such as those of the color of a red rose, the caw of a crow, the smell of a skunk, heat from a fire, the taste of chocolate, the pain of a kidney stone, the shape of a tennis ball.
      • Reflection includes thoughts, emotions, feelings, willings, urges, and desires.

Scientific Common-sense View of Perception

  • The scientific common-sense view of perception is that, under normal conditions, we see things as they are,
  • When a person sees a tomato, for example, light of wavelength 700 billionths of a meter bounces off the tomato, enters the eye, and hits the retina. The optic nerve transmits signals to the visual cortex at the back of the brain, resulting in a visual experience of a tomato.
  • The visual experience resembles the material tomato.

Locke’s View

  • Following Galileo, Locke distinguished between
    • Primary Qualities: size, shape, mass, movement.
    • Secondary Qualities: color, smell, taste, sound.
  • Locke argued that Primary Qualities are real attributes of physical objects but Secondary Qualities are not.
    • “The ideas of primary qualities of bodies are resemblances of them, and their patterns do really exist in the bodies themselves, but the ideas produced in us by these secondary qualities have no resemblance of them at all.” (Book II, Chapter 8, 15)
  • Primary Qualities
    • The weight we feel when we heft a tomato is caused by its mass
    • The tomato’s weight thus resembles the physical tomato.
  • Secondary Qualities
    • A tomato’s color is caused by light reflected off its surface, entering the eye, and reacting with photoreceptor cells in the retina, which triggers nerve impulses that travel to the visual centers of the brain, resulting in the experience of a red tomato.
    • The tomato’s color thus doesn’t resemble the physical tomato.
    • Rather, the brain projects redness onto the tomato we see.
  • Like the atoms making them up, physical objects have size, shape, and mass, but no color, smell, or taste.
  • The redness of a tomato, the smell of coffee, the taste of chocolate, and the sound of thunder are sensations produced, not by qualities in objects resembling them, but by the interaction of our sense organs with light waves, molecules, and sound waves coming from the objects.

Locke’s Arguments

  • Philosophers back up their views with arguments.
  • Locke presented three arguments that the secondary qualities we experience don’t resemble the physical objects that cause them:
    • Analogy with Pain
    • Porphyry in the Dark
    • Relativity of Sense Perception
  • Analogy with Pain
    • A person near a fire feels warmth. If you get too close, you also feel pain.  Why think “that this idea of warmth, which was produced in [you] by the fire, is ACTUALLY IN THE FIRE; and [your] idea of pain, which the same fire produced in [you] the same way, is NOT in the fire?” (Essay BK I #16)
  • Porphyry in the Dark
    • Porphyry is a red rock with white dots. (Photo below)  Turn off the lights and the colors disappear. “Can any one think any real alterations are made in the porphyry by the presence or absence of light; and that those ideas of whiteness and redness are really in porphyry in the light, when it is plain IT HAS NO COLOR IN THE DARK?” (Essay BK I #19)
  • Relativity of Sense Perception Argument
    • By heating one hand and cooling the other, you can stick both hands in a bucket of water so one hand feels warm and the other cool. If warmth and coolness were real qualities of water, the water would be simultaneously both warm and cool, which is impossible.

Opening the Door for Berkeley, Hume, and Kant

  • Locke distinguished between the experience of seeing a red tomato and the causal explanation of the experience; and argued that, for secondary qualities, the two don’t match.
  • The distinction opened the door for the more radical views of Berkeley, Hume, and Kant.

Addendum: Demonstrative and Probable Reasoning

  • Book IV, Chapter XV. Of Probability
    • As DEMONSTRATION is the showing the agreement or disagreement of two ideas, by the intervention of one or more proofs, which have a constant, immutable, and visible connexion one with another; so PROBABILITY is nothing but the appearance of such an agreement or disagreement, by the intervention of proofs, whose connexion is not constant and immutable, or at least is not perceived to be so, but is, or appears for the most part to be so, and is enough to induce the mind to judge the proposition to be true or false, rather than the contrary.
    • For example: in the demonstration of it a man perceives the certain, immutable connexion there is of equality between the three angles of a triangle, and those intermediate ones which are made use of to show their equality to two right ones; and so, by an intuitive knowledge of the agreement or disagreement of the intermediate ideas in each step of the progress, the whole series is continued with an evidence, which clearly shows the agreement or disagreement of those three angles in equality to two right ones: and thus he has certain knowledge that it is so.
    • But another man, who never took the pains to observe the demonstration, hearing a mathematician, a man of credit, affirm the three angles of a triangle to be equal to two right ones, assents to it, i.e. receives it for true: in which case the foundation of his assent is the probability of the thing; the proof being such as for the most part carries truth with it: the man on whose testimony he receives it, not being wont to affirm anything contrary to or besides his knowledge, especially in matters of this kind: so that that which causes his assent to this proposition, that the three angles of a triangle are equal to two right ones, that which makes him take these ideas to agree, without knowing them to do so, is the wonted veracity of the speaker in other cases, or his supposed veracity in this.