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Lecture 5

PHLA10H3 Lecture Notes - Lecture 5: Reductio Ad Absurdum, Possible World

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William Seager

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September 22, 2015
Philo Lec 05: Philosophy of Religion
Faith and Reason
Philosophy of religion
oNature of religion
Why does religion exist?
Why religions geographically distributed?
Why are there so many different religions
oBasic questions
Does God exist? #1 question
What is the nature of God?
What evidence for God?
(metaphysics questions about the nature of things)
Faith and Reason
oPhilosophy looks for reasons through arguments
oArguments not authority; authority is useless to us
oFaith is not a philosophical argument
The Five Ways of Aquinas
Aquinas is a medieval theologian and philosopher who, in his book, presents five proofs
that God exists. He called them the “Five Ways”
Each of his arguments begins with a simple observation that is supposed to be obvious
to everyone. Each argument then proceeds through various other premises to the
conclusion that there is a God
oAquinas intends each of his proofs to be deductively valid
The Concept of God
According to Aquinas, God is …
oA person: you have a personal relationship with God
oOmnipotent: God is all-powerful. There is nothing He can’t do because he’s a
Superior being
oOmniscient: God is all-knowing. There is nothing He does not know. He has all
the knowledge in the world
oOmni-benevolent: God is entirely good. He has no weaknesses and no evil in
A Logical Problem
oTrick question: Can God make a stone so heavy that He can’t lift it?
oIf so, then his power is limited. If not, then his power is limited
oAre “logical limitations” a genuine limitation of God’s power?
oDescartes claimed that he can’t explain how God can lift the stone, because
that’s beyond the limitations of human knowledge
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Way 1: Motion
In the natural world, some objects move. Any movement requires a cause, which must
come before their effects. An infinite chain of causes is impossible, so there has to be a
first cause of motion.
This first cause cannot be another moving body, so it is something “outside” the physical
world – perhaps God
Way 2: General Causation
No event occurs unless it has a cause. Causes must occur before their effects
An infinite chain of causes is impossible. Therefore, there is a first event, “A”, which
must have a cause, but it cannot be another event in the world
There must be something outside of this world that causes the first event “A” – perhaps
Issues with the first 2 ways
Some premises are questionable
Consider this premise from the 1st way: any movement requires a cause
oAristotelean vs. Newtonian physics
Aristotle believed Aquinas’ premise
Not until Galileo/Newton was it shown to be false
Newton’s First Law: a body at rest remains at rest, and a body in motion
continues to move in a straight line with a constant speed unless and
until an external unbalanced force acts upon it
oIt is easy to repair this problem: replace motion with acceleration
Every change in motion of a body requires a cause
But then, what about this premise: causes must come before their effects
oIs “backwards causation” impossible? Perhaps it is at least unlikely
oWhat about Aquinas’ inference from “there is a cause outside nature” to “the
cause outside nature is God”?
How can we prove that the “outside cause” has all the features of God?
The Birthday Fallacy
Aquinas assumes that this argument is valid: Every event has a cause.
Therefore, there is a cause to every event.
This is a counterexample, an invalid argument
Examples of counterexamples like this one: Everyone has a birthday.
There is one day that is everyone’s birthday. Every person has a father.
Therefore, there is someone who is everyone’s father. Every wife has a
husband. Therefore, there is someone who is the husband of every
This suggests that there could possibly be a whole bunch of causes of
the events in our world instead of only one
Infinite Time
oAquinas uses the premise that there cannot be an infinite chain of causes in the
past. But why not?
oEven if we assume that no event in nature can happen without having its cause,
it does not follow that there has to be a first natural event
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