According to Descartes, you cannot doubt that you are a thinking thing. If you deny it, you are contradicting yourself in the very act of denying it.

Methodological Scepticism/

Method of Doubt—assuming a skeptical position to prove a point.

Dream Arg. (External Scepticism)

P1—A posteriori beliefs come from external experience.

P2—Dreams are an internal experience.

P3—Subjectively, I cannot tell the difference between a dream and an external experience because I could just be dreaming about an external experience.

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Therefore, I can doubt my beliefs about external experience and the world.

This shows we may not know that an external world exists.

Evil Genius Arg. (Internal Skepticism)

P1—A priori beliefs do not rely upon experience and are based on reason.

P2—A priori beliefs are true by definition.

P3—A subtle, clever Evil Genius exists that can deceive you about a priori beliefs.

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Therefore, even a priori beliefs can be false.

At this point, both types have been doubted.

So the Method of Doubt takes you through two types of beliefs, causes you to doubt both of them and you end up with something that you cannot doubt. From there Descartes starts constructing his philosophical system.

P1—I think, therefore I exist.

So, it is important to understand Descartes wasn’t a true skeptic like David Hume or others who doubt certainty is possible. Instead, Descartes used skepticism to prove his point.

David Hume’s Argument Against Induction

Hume followed Descartes in time and you can see Hume’s argument here as an attack on Cartesian philosophy in two senses. Descartes thought that reason ruled and that certainty is provable. Hume denies both of these.

Induction—for Hume, reasoning from cause to effect, or what is observed to what is unobserved

Hume’s Argument Against Induction

PUN--The principle of the uniformity of nature states that the future will be like the past. PUN is an inductive principle.

P1—PUN cannot be proven deductively. (We would need a description of the complete state of nature)

P2—PUN cannot be proven inductively. (You cannot prove an inductive principle by appealing to induction.)

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C-Therefore PUN cannot be proven and we have no reason to suppose the future will be like the past.

Notice the conclusion, Hume is not saying that we don’t believe, he is saying that we don’t have a good reason or any reason to believe it. Hume was a skeptic about reason.

Problem of Other Minds

P1—Subjectively I know I have mind.

P2—Objectively, I don’t know if you have a mind.

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C-Therefore, I don’t know if other minds exist like my own.

One way it is ‘solved.’

P1—Subjectively I know I have a mind.

P2—Objectively, I do know you have a mind.

P3—The mind is not something

‘inner’ but something outer that can be observed like language and behavior.

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C-Therefore other minds exist.

Notice what happens in the solution, an objective criteria of behavior/language is substituted for a subjective one.

Does this really solve the problem?

How to Solve or Postpone Skepticism

Wittgenstein believed that we have certain concepts that irreducible and themselves cannot be verified. Things like time and space, for instance, are basic concepts and other concepts rely upon them. It make no sense to question them because they are needed for intelligibility or to have any discourse at all. Why aren’t they skeptical about their word meanings?

In the solution to the problem of other minds, intelligent discourse shows concepts known and used in a way that is not simply mechanical. If that is true, then ‘mind’ might best be thought of as competency with concepts. The inner becomes the outer.