Descartes and Russell
Posted: Wed, Oct 8, 2025
Three levels of Cartesian doubt
What can I be certain of?
| Level of doubt | Reason for doubt | Scope of doubt |
|---|---|---|
| 1st | The senses sometimes deceive us. | I can’t be certain of beliefs acquired through senses. |
| 2nd | Being awake and being asleep are indistinguishable. | I can’t be certain of pretty much anything other than mathematical truths. |
| 3rd | I could be deceived by an evil demon. | Can I be certain of anything? |
Descartes:
I will suppose . . . some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me. I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgement. I shall consider myself as not having hands or eyes, or flesh, or blood or senses, but as falsely believing that I have all these things. (AT 23)
This is a skeptical scenario where appearance (how things seem to be) comes apart from reality (how things in fact are).
- Is there any good basis for believing that we do not live in a skeptical scenario?
- Descartes: Cogito, ergo sum; I think, therefore I am.
Russell’s strategy: Inference to the best explanation (IBE)
Russell: IBE offers us good reason for believing, even if it does not prove, that we are not in a skeptical scenario.
Observations: It hasn’t rained; the grass is wet.
- Hypothesis I: It rained.
- Hypothesis II: The sprinklers came on.
- Hypothesis III: Grass does not get wet on its own without being rained on except at BC where laws of nature do not apply.
- Hypothesis IV: Aliens visited BC to play water pistols.
Some theoretical virtues:
- Explanatory power: How well does the hypothesis render the observations predictable/expectable?
- Parsimony: How many principles and entities does the hypothesis postulate in order to do this? (“Ockham’s razor”)
- Consistency: How well does the hypothesis avoid contradicting itself?
- Unity: How well does the hypothesis avoid ad hoc elements that don’t integrate into a whole?
- Conservatism: How well does the hypothesis cohere with already-established theories?
Observations: There appear to be tables, chairs, …
- Real-world hypothesis: There are tables, chairs, …
- Skeptical hypothesis: Say, the Matrix.
Russell: The real-world hypothesis should be preferred because it is more “simple” or less elaborate.
- Is it?