Notes on Descartes' Meditations

Sandra LaFave

A priori and a posteriori

(This is review, but some students need to read this again!)

The expressions “a priori” and “a posteriori” are Latin idioms. They are phrases and function as adjectives or adverbs. That is, they modify a noun (such as “knowledge,” “statement,” or “claim”), a verb (such as "know") or an adjective (such as “true” or “false”). The “a” in these expressions is the Latin preposition meaning “from.”  So “a priori” means “from before [observation]” and “a posteriori” means “from after [observation]”. The expressions “a priori” and “a posteriori” describe how we know the truth or falsity of a statement.

A statement is true or false a priori if no observation or experiment is required to determine if it is true or false. Examples of a priori statements are mathematical assertions, statements true or false by definition, and logical truths and falsehoods. We “just know” when some claims are a priori true or false. For example, we “just know” that the same statement cannot be both true and false in the same sense at the same time (a rule of logic called the law of non-contradiction).

A statement is true or false a posteriori if observation or experiment is required to determine if it is true or false; we don’t “just know” it. Examples of a posteriori statements are statements about the world, e.g., “Dogs are carnivores” or “Ottawa is the capitol of Canada.”


Meditation I

Descartes describes his method of systematic doubt (aka “methodological doubt”): he will claim to know only that which he cannot doubt (that which is indubitable). Descartes succeeds in doubting both a posteriori and a priori statements, in the following order:

  1. a posteriori statements:

    • "My senses are reliable."

      Descartes wonders how he "really knows" this; he asks if his senses may not be systematically deceiving him.

    • "I am awake."

      Descartes asks how he really knows this, and wonders if he may not be dreaming.

  2. a priori statements:

    • "Mathematical statements (such as the statement "2+3=5") are true."

      Descartes realizes that while such statements seem indubitable, they might seem indubitable only because an all-powerful evil deceiving God is making them seem indubitable.

    • "God is not a deceiver."

      Of course, the ultimately clever deceiver God would want me to think this ...

Note that Descartes can doubt math only by supposing God to be a deceiver. If God is not a deceiver, in other words, math is indubitable.


The Overall Argument of the Meditations

The general strategy of the Meditations can now be stated. If Descartes can prove that God exists and is not a deceiver, math knowledge will be restored. If math knowledge is restored, then Descartes can attain knowledge of the external world (if there is one) as that world is known through math, i.e., knowledge of its measurable, objective qualities. Math is the language of science. So if math can be known, scientific knowledge really is genuine!

A lot of our ordinary experience isn't science, though. We "just feel" a lot of stuff, e.g., pleasure and pain, and our feelings are private. Science can't access our private feelings to measure them, so the certainty of math doesn't help here. Pleasure, in particular, creates moral challenges for people. The subjective non-mathematical sensory world is still potentially deceptive; what feels right isn't always what is right. This is where the Church comes in. Since many of people's sense experiences are still liable to be deceptive, people will forever be in need of guidance and correction by the Church.

In this way, Descartes convinces the Church that it has nothing to fear from science, since scientists themselves, whether they know it or not, have to trust God for their mathematically certainty. Furthermore, the Church's crucial role in areas of faith and conscience remains exactly the same: the Church still rules in areas of ethics and religion.

Thus Descartes reconciled the Church and the new science — so much so that the Catholic Church never seriously quarreled with science again. Catholics have never opposed evolution, for example, and Catholic universities have the same science curricula as secular universities.

Let's continue with the details of Descartes' argument, then.


Meditation II

Descartes realizes he has one indubitable idea that withstands even the evil demon conjecture: that he exists (which remains true even if he is being deceived).  “I think, therefore I am” — cogito ergo sum. "Cogito" is Latin for "I think"; "ergo" is Latin for "therefore"; "sum" is Latin for "I am".

--- Take a break ---

Do you know the Beach Boys' song "In My Room"? Philosopher Alan White has written new words to the same tune. Click here, wait for download, and listen online. Here are the words:

Ergo Sum
(Sung to the Beach Boys' In My Room)

Descartes sat and meditated,
and thus cogito'ed --
ergo sum, ergo sum.
Doubting all he took as granted
forced him to conclude --
ergo sum, ergo sum.

It took four more meditations
for the world to bloom --
proving God deposed deception
(with distinctness too!).

The path of all modern thinking
starts with Descartes' turn --
ergo sum, ergo sum...

--- End of break ---

Descartes now knows he exists, but only as a thinking thing, and not as a body. Descartes believes that a person is made up of two separate and distinct kinds of substance — material substance (called res extensa) and thinking substance (res cogitans). (Descartes assumes bodies can't think. Is this a reasonable assumption?) The view that reality comprises two separate and incompatible kinds of substance has come to be known as Cartesian dualism.

Cartesian dualism gives rise to the notorious mind-body problem — the problem of how to explain exactly how these two kinds of incompatible and opposite being interact in a person. We will explore alternatives to Cartesian dualism in the materials for Quiz 7.

The argument of the wax shows Descartes that he has other indubitable ideas also. He knows that wax is the same although all its sense data change: that this substance is continuous (the same) through time. His sense data (which have completely changed) do not tell him this. He “just knows” substantiality (thingness) and sameness (identity).

Thus by the end of Meditation II, Descartes thinks he knows three things for certain: (1) his own existence qua thinking thing, and the concepts of (2) substance and (3) identity. He still does not know if any substances exist.


Meditations III - V

Descartes gives arguments that attempt to prove the existence of a non-deceiving God. Since Descartes still isn’t sure there is anything outside his mind, his arguments for God must be completely a priori, i.e., not derived from any facts about the cosmos. The most important of Descartes’ arguments for God is the Ontological Argument in Meditation V, which we will analyze later in the semester.

If God is not a deceiver, then Descartes gets math knowledge back — note, math knowledge only, not ordinary non-mathematical sense knowledge. He needs a separate argument to show that he can trust his senses. He gives that argument in Meditation VI.


Meditation VI

Here Descartes gives his argument that he is distinct from his body and can exist without it (thus proving that personal immortality is possible). Here’s the argument: Descartes knows clearly and distinctly of himself that he is a thinking thing; yet his idea of body is clearly different. In fact, his idea of body is incompatible with thinking; in Descartes’ mind, the idea of “thinking body” is simply self-contradictory. (Is this argument convincing?)

Next Descartes shows that material (“corporeal”) things do exist and that he can therefore trust his senses to some degree. But Descartes will not abandon the method of systematic doubt even at this late point in the argument; he will allow only what he cannot doubt, and he sees that the ordinary senses are still liable to error. For example, we see a white object as red if a red light is illuminating it; we may fail to savor the taste of food if we have a bad cold. He concludes that he can know the material world with certainty only as it is ”the object of pure mathematics.” Descartes introduces a distinction between objectively measurable properties of things (like size, shape, and weight – what John Locke will call primary qualities) and “subjective” non-measurable properties (like color, smell and taste – what Locke will call secondary qualities). We can only really know the objective, quantifiable stuff. This just happens to be exactly the stuff that is vital for the new mathematically-grounded sciences.

Descartes’ conclusion made everyone happy: the Church had nothing to fear from science.

  1. Subjective sense experience is still liable to error, so people still need the Church. The Church rules in the realm of SOUL.

  2. Science rules in the realm of BODY — external objects as measured and quantified by math — but only because the certainty of science depends on the existence of a non-deceiving God. Whether they admit it or not, scientists depend on God.




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