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PHIL 1F91 Lecture Notes - Modus Tollens, Deductive Reasoning, Logical Form

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Brian Lightbody

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PHIL 1F91 September 21, 2012
Lecture Three: Judging and Evaluating Arguments
- We now come to the fourth component of philosophical methodology which has to do with the
- As we already know, philosophers first clarify their concepts and definitions. They then set up
and hypothesis to test the concept. Finally, they construct an argument to support their
- Philosophical texts put forward arguments to support their conceptual theories
- First, we may notice that there are two different meanings for the word „argument‟ in English.
The first definition would be what Rauhuat calls a “verbal disagreement”
- Ex. Someone hits you from behind in your new car. You engage in an “argument” with this
- A philosophical argument is somewhat different. Every philosophical argument is comprised of
a premise or set of premises and a conclusion. We will define an argument is a series of
sentences, one of which (the conclusion) is claimed to be supported by the others (premises)
- 1. If it rains then the streets will be wet (premise)
- 2. It rained (premise two)
- Therefore: the streets are wet (conclusion)
- The first part of this argument (the if clause) is called the ANTECEDENT. The second part
(the then clause) is called the CONSEQUENT
*The above argument has been put into „standard form”- the premises are numbered and the conclusion is
clearly stated. As Rauhut mentions, sometimes it is difficult to put an argument into standard form. But
doing so is absolutely essential in order to clarify the argument.
Examples of Difficult Arguments:
- There is no past or future even in thought. Because to think it you have to make it present.
- See how foolish it is to say: I should prefer nonexistence to miserable existence. He who says,
I prefer this to that, chooses something. Nonexistence is not something; it is nothing. There
can be no real choice when what you choose is nothing.
Deductive Arguments vs. Inductive Arguments
Example of a Deductive Argument:
1. The world is structured, ordered and beautiful
2. Anything that is structured, ordered and beautiful must have been designed by someone to be all
of these things
3. Therefore: the world was designed.
4. The above is VALID (structure or form) arguments because it has the following structure:
- W is B
- All B‟s are D‟s
- Therefore W is D‟s

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PHIL 1F91 September 21, 2012
- A deductive argument is valid if it satisfies the following conditions: if all the premises of the
argument are presumed to be true then the conclusion must be true as well
Example of an Inductive Argument
1. All students has rich parents
2. Peter is a student
3. Therefore peter has rich parents.
4. Therefore: Peter has rich parents
This argument is invalid because it is possible for the premises to be true and the conclusion false. This is
the very definition of INVALIDITY
Valid vs. Invalid arguments Cont‟d
Valid arguments guarantee that the conclusion must follow if we assume that they premises
are true. With invalid arguments there is no guarantee that the conclusion is true even if we
assume that the premises are true.
Validity is simply a necessary condition that deductive arguments must satisfy in order to be
considered materially true.
Validity and soundness
Finally we have what philosophers call soundness. An argument is sound if the argument is valid and the
premises are true in reality. Philosophers are always trying to establish sound arguments
all lions are cats
All cats are mammals
Therefore all lions are mammals
*The arguments are valid and the premises are materially true. The argument is therefore sound.
Sound Arguments
1. If all events are caused, then we are not free
2. All events are caused.
3. Therefore we are not free
This argument is valid and appears to be sound. Yet many of us may appear to be sound. Yet many of us
may disagree with the conclusion. At the end of this section, I will show you how we can refute this
Inductive Arguments
Finally we have inductive arguments. Inductive arguments can be defined as: an argument that is not
deductively valid but whose premises purport to provide some measure of support for its conclusion.
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