David Hume's Reply to Descartes

The Ultimate Skeptic is Rebuffed by the Ultimate Empiricist

Sep 20, 2009

Descartes wanted to be absolutely sure of something that he couldn't doubt, so he said, "I think therefore I am." David Hume replied with a model to undo Descartes doubt.

Rene Descartes (1596-1650), the mathematician who invented Analytic Geometry, was also a philosopher who wanted to know what was real and what was not. David Hume (1711-1776), the British Empiricist, looked at Descartes model of the mind and took it apart.

Descartes Model of the Mind

The inventor of Analytic Geometry, Descartes, was also interested in knowledge, how we know what we know; this is epistemology. So he started out doubting everything because he was only sure of himself and his feelings. He concluded that only knowing himself, that he was thinking, was the only thing that he could not doubt.

His next step was to slowly reconstruct the what made the mind recognize ideas. He modeled the mind as a receiver of external sensory objects. This included colors, sounds, weather and temperature variations, bright lights, and living objects. If one saw a mountain in the distance, it was immediately impressed on the mind as an idea. The mind can only recognize ideas; these external world images are embedded internally in the mind.

Descartes' philosophy also held that the mind is a passive receiver. It accepts what ever happens to cross its path. This also includes ideas like geometry, or language, in fact it is anything that the mind works with.

Philosopher David Hume

David Hume was a Scottish philosopher, often considered the last of the great three thinkers, which includes John Locke and Bishop Berkley. Having accepted Descartes' model of the mind, David Hume asked one important question how does cause and effect work? Consider that there are at least two joint ideas that make cause and effect appear in the mind, so how does that work?

For example, if a cup falls from a table and breaks, the effect is the breaking of the cup, and the cause is the cup falling. This event is seen all the time. One naturally concludes that one event affects the other; that one idea follows the other. But as ideas go, the two ideas are separate from one another. One of the features of cause and effect is the necessary connection between the two ideas. One event necessarily causes the other. But Hume was bothered by Descartes characterization of the idea of cause and effect. Hume applied formal logic to address this problem. At his disposal were the syllogisms of modus ponens and modus tollens.

Logic - Modus Ponens

One way to look at this is by using the logical argument called modus ponens. Simply put, that means IF P (is true) then Q(is true); now P (is true) therefore Q(is true). In shorthand If P-> Q, P therefore Q.

A logical fallacy occurs if one does not use modus ponens correctly. This is an incorrect application of Modus Ponens. If P -> Q, Q is true, therefore P is true. That is incorrect. In other words, through an example. If the Cowboys are won the Playoffs then the Cowboys are in the Superbowl. (In this example there are more than playoff games possibilities so there is no logical connection between winning a playoff game and entering the Superbowl.)

Logic - Modus Tollens

The next example of logic is modus tollens. It starts like modus ponens: If P then Q, but it uses the negative; not Q, therefore not P. The full statement is this: If P then Q, not Q therefore not P.

If the Cowboys are in the Superbowl, then they won the Playoff championship game. The Cowboys did not win the playoff championship game, therefore they did not go to the Superbowl.

Hume's Rebuff of Descartes Model of the Mind

David Hume, in the book, The Treatise of Human Nature, took Descartes model and asked, what is the necessary connection between cause and effect. He concluded that according to Descartes, there was none. If one looks at the cup and the subsequent broken pieces on the floor, the two ideas are separate and independent. Hume says that there is no logical connection. But that is just the start.

Next Hume uses modus tollens to attack Descartes model. The argument is that if the mind is filled with ideas from the external world and one of those ideas is cause and effect. But there has to be a necessary connection between the cause and the effect. Descartes' model does not show how this works or how it would work. So the model cannot explain cause and effect because the necessary connection that joins the two ideas is missing, therefore the model is incorrect. That is modus tollens.

If P then Q, not Q then not P. The mind as just a passive receptacle was incorrect. A new model of the mind was necessary.

David Hume's attack on Descartes set off a different direction in philosophy, culminating with Immanuel Kant's Critique of Pure Reason. Empiricism was now the most important epistomological philosophy.

The copyright of the article David Hume's Reply to Descartes in Western Philosophy is owned by George Garza. Permission to republish David Hume's Reply to Descartes in print or online must be granted by the author in writing.