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SKEPTICISMSection 3
Three Theories of Knowledge
Three Theories of Knowledge
1. Skepticism
Three Theories of Knowledge
1. Skepticism This is the view that we cannot justify our
beliefs
Three Theories of Knowledge
1. Skepticism This is the view that we cannot justify our
beliefs;And therefore that knowledge is
impossible.
Three Theories of Knowledge
1. Skepticism 2. Empiricism
This is the view that our beliefs can be justified if they agree with the evidence provided by our senses.
Three Theories of Knowledge
1. Skepticism 2. Empiricism3. Rationalism
This is the view that our beliefs can be justified if they are sufficiently rational.
Three Theories of Knowledge
1. Skepticism: We can know nothing.2. Empiricism:3. Rationalism:
Three Theories of Knowledge
1. Skepticism: We can know nothing.2. Empiricism: We know what we see,
touch, etc. 3. Rationalism:
Three Theories of Knowledge
1. Skepticism: We can know nothing.2. Empiricism: We know what we see,
touch, etc. 3. Rationalism: We know what makes
sense according to reason.
Skepticism
From the Greek Word
ske,yijPronounced skepsis, meaning: inquiry.
Skepticism
• Skepticism comes in two forms:
Skepticism
• Skepticism comes in two forms:1. Local Skepticism and
Skepticism
• Skepticism comes in two forms:1. Local Skepticism and2. Global Skepticism
Global Skepticism
• Global Skepticism is the view that we can know nothing at all about anything.
Global Skepticism
• Global Skepticism is the view that we can know nothing at all about anything.
• “Global” means that it is universal.
Global Skepticism
• Global Skepticism, boldly stated, is not a serious position.
Counter Argument
1. If global skepticism is true, then no one can know anything.
Counter Argument
1. If global skepticism is true, then no one can know anything.
2. If no one can know anything, then no one can know that global skepticism is true.
Counter Argument
1. If global skepticism is true, then no one can know anything.
2. If no one can know anything, then no one can know that global skepticism is true.
Therefore: if global skepticism is true, then no one can know that it is true.
Counter Argument
1. If global skepticism is true, then no one can know anything.
2. If no one can know anything, then no one can know that global skepticism is true.
Therefore: if global skepticism is true, then no one can know that it is true.
• Quick: what is the form of that argument?
Counter Argument
1. If global skepticism is true, then no one can know anything.
2. If no one can know anything, then no one can know that global skepticism is true.
Therefore: if global skepticism is true, then no one can know that it is true.
• Quick: what is the form of that argument?
• Answer: hypothetical Syllogism
1. If p, then q;2. If q, then r; Therefore: if p then r.
Local skepticism
• Local skepticism is the view that we can’t know anything about some particular subject.
– The afterlife.– Life on other planets.– God.
Rene Descartes
From local skepticism back (almost) to global skepticism.
1. Sometimes my senses deceive me. Therefore: my senses are unreliable.
Rene Descartes
1. Sometimes my senses deceive me.
Therefore: my senses are unreliable.
Rene Descartes
1. Sometimes my senses deceive me.
Therefore: my senses are unreliable.
• What kind of argument is this?
Rene Descartes
1. Sometimes my senses deceive me.
Therefore: my senses are unreliable.
• What kind of argument is this?
• An enumerative inductive argument.
Rene Descartes
1. Sometimes my senses deceive me.
Therefore: my senses are unreliable.
• What kind of argument is this?
• An enumerative inductive argument.
• Is it strong?
Rene Descartes
1. Sometimes my senses deceive me.
Therefore: my senses are unreliable.
• What kind of argument is this?
• An enumerative inductive argument.
• Is it strong?• No.
Descartes’ Dream
1. If it is possible that I am dreaming right now, then I cannot be certain that any of my perceptual beliefs is true.
Descartes’ Dream
1. If it is possible that I am dreaming right now, then I cannot be certain that any of my perceptual beliefs is true.
2. It is possible that I am dreaming right now.
Descartes’ Dream
1. If it is possible that I am dreaming right now, then I cannot be certain that any of my perceptual beliefs is true.
2. It is possible that I am dreaming right now.
Therefore: I cannot be certain that my perceptual beliefs are true.
Descartes’ Dream
1. If it is possible that I am dreaming right now, then I cannot be certain that any of my perceptual beliefs is true.
2. It is possible that I am dreaming right now.
Therefore: I cannot be certain that my perceptual beliefs are true.
• Classic modus ponens.
1. If p, then q.2. p.Therefore q.
Descartes' Dream
• Do we know when we are awake and when we are dreaming?
Descartes' Dream
1. I cannot always tell when I am dreaming.
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
3. If I can’t be certain I am not dreaming now, then I cannot trust my perceptual beliefs.
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
3. If I can’t be certain I am not dreaming now, then I cannot trust my perceptual beliefs.
Therefore: I cannot trus my perceptual beliefs.
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
3. If I can’t be certain I am not dreaming now, then I cannot trust my perceptual beliefs.
Therefore: I cannot trus my perceptual beliefs.
• What form of argument is this?
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
3. If I can’t be certain I am not dreaming now, then I cannot trust my perceptual beliefs.
Therefore: I cannot trus my perceptual beliefs.
• What form of argument is this?
• It is a combination of two forms:
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
3. If I can’t be certain I am not dreaming now, then I cannot trust my perceptual beliefs.
Therefore: I cannot trus my perceptual beliefs.
• What form of argument is this?
• It is a combination of two forms:– Modus Ponens and
Descartes' Dream
1. I cannot always tell when I am dreaming.
2. If I can’t always tell, then I can’t be certain I am not dreaming now.
3. If I can’t be certain I am not dreaming now, then I cannot trust my perceptual beliefs.
Therefore: I cannot trus my perceptual beliefs.
• What form of argument is this?
• It is a combination of two forms:– Modus Ponens and– Hypothetical
Syllogism
The Brain in a Vat Argument
• A stronger version of Descartes’ dream
1. My brain receives all its information about the world through signals traveling on my nervous system..
The Brain in a Vat Argument
1. My brain receives all its information about the world through signals traveling on my nervous system.
2. It is conceivable that a computer could recreate those same signals and transmit them along my nervous systems.
The Brain in a Vat Argument
1. My brain receives all its information about the world through signals traveling on my nervous system.
2. It is conceivable that a computer could recreate those same signals and transmit them along my nervous systems.
Therefore: it is possible that the world I perceive is entirely created by a computer.
The Brain in a Vat Argument
• If you brain were kept alive in a vat of liquid;
The Brain in a Vat Argument
• If you brain were kept alive in a vat of liquid;
• And a computer were feeding you the same input that you are getting now.
The Brain in a Vat Argument
• If you brain were kept alive in a vat of liquid;
• And a computer were feeding you the same input that you are getting now.
• Could you tell the difference?
The case for skepticism
• We can’t know that global skepticism is true.
The case for skepticism
• We can’t know that global skepticism is true.
• But we haven’t found anything we can be certain about.
The case for skepticism
• We can’t know that global skepticism is true.
• But we haven’t found anything we can be certain about.
• That’s as good as knowing that we can’t know anything.
