You Know Nothing: Dreaming

David Sanson

2026-10-07

Preliminaries

Agenda

1. Preliminaries
2. Review: Knowledge of the Future
3. Knowledge of the Present

Reminders

• We are discussing the second part of “You know nothing”.
• I divided things up differently this week:
  • Monday: you cannot know about the future
  • Wednesday: you cannot know about the present
• I returned exams on Monday. You can pick yours up at the end of class.

Review: Knowledge of the Future

Agenda

1. Preliminaries
2. Review: Knowledge of the Future
3. Knowledge of the Present

Induction and FLP

Argument

1. In the past, the sun has always set in the west.
2. So, the sun will set in the west tonight.

Future Like Past (FLP)

Future states of the world will be like past states of the world.

Without FLP, (1) would not be evidence for (2).

Against Knowing the Future

Argument

• KF1. If you are not justified in believing that FLP is true, then your belief that the sun will set in the west tomorrow is unjustified.
• KF2. You are not justified in believing that FLP is true.
• KF3. So, your belief that the sun will set in the west tomorrow is unjustified.
• KF4. If your belief that the sun will set in the west tomorrow is unjustified, then you don’t know that the sun will set in the west tomorrow.
• KF5. So, you don’t know that the sun will set in the west tomorrow.

KF4: Knowledge requires justification. You are justified in believing something when you have a good reason to believe it.

KF1: Your belief that the sun will set in the west tomorrow is based on FLP.

KF2: Your belief in FLP is unjustified

Argument

• UJ1. If your belief in FLP is justified, then it is either justified by direct observation or by inductive reasoning.
• UJ2. Your belief in FLP isn’t justified by direct observation.
• UJ3. Your belief in FLP isn’t justified by inductive reasoning.
• KF2. So, your belief in FLP is unjustified.

UJ1: There are other sources of knowledge—introspection, mathematical proof. But these won’t get you knowledge of the external world.

UJ2: You cannot directly observe the future.

UJ3 and Circularity

Argument

• AC1. All inductive reasoning about the future assumes the truth of FLP.
• AC2. If all inductive reasoning about the future assumes the truth of FLP, then any inductive reasoning about FLP is circular.
• AC3. No belief can be justified by circular reasoning.
• UJ3. So, FLP isn’t justified by inductive reasoning.

Knowledge of the Present

Agenda

1. Preliminaries
2. Review: Knowledge of the Future
3. Knowledge of the Present

Dreaming

The Dreaming Hypothesis (TDH)

You are currently lying down in bed dreaming about being in philosophy class.

The Dreaming Argument

Argument

• DR1. If you have no way of knowing that TDH is false, then you don’t know that you’re sitting down reading.
• DR2. You have no way of knowing that TDH is false.
• DR3. So you don’t know that you’re sitting down reading.

The Competing Hypotheses Argument

Argument

• CH1. One knows a certain thing only if one has some way of knowing that all competing hypotheses are false.
• CH2. TDH is a hypothesis that competes with your belief that you’re sitting down reading.
• DR1. So, if you have no way of knowing that TDH is false, then you don’t know that you’re sitting down reading.

CH1: You don’t know that the bird you see is a hawk if you cannot rule out the hypothesis that it is an eagle or a falcon.

The Argument from Deduction

Argument

• DE1. If you know you’re sitting down reading, then you can deduce that TDH is false from things you know.
• DE2. If you can deduce something from things you know, then you have a way of knowing that thing.
• DE3. So, if you know you’re sitting down reading, then you have a way of knowing that TDH is false.

Deducing that TDH is false

1. I’m sitting down reading.
2. If I’m sitting down reading, then I’m sitting.
3. If I’m sitting, then I’m not lying down.
4. If I’m not lying down, then I’m not lying down dreaming.
5. If I’m not lying down dreaming, then TDH is false.
6. So, TDH is false.

Contraposition: DR1 and DE3

These are equivalent, because ‘if P then Q’ is equivalent to ‘if not Q then not P’:

Argument

• DE3. If you know you’re sitting down reading, then you have a way of knowing that TDH is false.
• DR1. If you have no way of knowing that TDH is false, then you don’t know that you’re sitting down reading.

• If you live in Normal, then you live in Illinois.
• If you live in Illinois then you live in Normal.
• If you don’t live in Illinois, then you don’t live in Normal.
• If you don’t live in Normal then you don’t live in Illinois.

The Dreaming Argument

Argument

• DR1. If you have no way of knowing that TDH is false, then you don’t know that you’re sitting down reading.
• DR2. You have no way of knowing that TDH is false.
• DR3. So you don’t know that you’re sitting down reading.

The No Evidence Argument

Argument

• NE1. If you have no evidence against something, then you have no way of knowing it’s false.
• NE2. You have no evidence against TDH.
• DR2. So, you have no way of knowing that TDH is false.

encourage discussion of the idea that there might be tests for whether or not you are dreaming.

The No Useful Tests Argument

Argument

• NU1. If you don’t know that a dreaming test is reliable, then you can’t know whether you’re dreaming by using it.
• NU2. You can never know that a dreaming test is reliable.
• NU3. So, you can never know whether you’re dreaming by using a dreaming test.

NU1: Spinning top example from Inception. If you don’t know this is a good test, you cannot know your not dreaming by using it.

NU2: All the apparent past evidence you might have that the test is reliable could have happened in a dream!