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Chapter 1

PHI 1101 Chapter Notes - Chapter 1: Enthymeme, Deductive Reasoning, Soundness


Department
Philosophy
Course Code
PHI 1101
Professor
Laura Byrne
Chapter
1

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PHI 1101 Critical Thinking book notes
Chapter 1
1. Inference: a relationship between 2 thoughts that occurs when one
thought provides support (/justifies) for another thought or makes it
reasonable to believe that another thought is true.
This relationship is signaled by INFERENCE INDICATORS. (e.g.
therefore, so… See CT P4)
Sometimes the inference indicator is missing: This can occur when a
speaker thinks the inference is quite obvious. For example:
It’s raiig; I’d better take y ubrella.
2. Statement: A sentence used to make a claim. Statements are capable
of being either true or false. Logic also calls them assertions or
propositions.
o Two Laws of Logic
1) The Law of Non-Contradiction: its impossible for both a
proposition and its negation to be true at the same time.
2) The Law of the Excluded Middle or the Law of Bivalence: every
proposition must be either true or false. In other words, any
middle position between truth and falsity is excluded.
3. Sets of Propositions Consistency-------additional concept
o Sets
Propositions can be combined in groups or sets.
e.g. Socrates is mortal. Socrates is a philosopher.
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2
o Consistency : a set of propositions is consistent if and only if it is
possible for all of the propositions in that set to be true at the
same time. In another words, a set of propositions is consistent if
these propositions do not contradict one another.
Concept-----Set
Property-----Consistent or inconsistent.
E.g. Consistent: Lassie is a dog. Lassie barks.
Inconsistent: Socrates is mortal. Socrates is an Olympian God.
4. Arguments: a set of statements that claims that one or more of those
statements, called the premises, support, or justify, or make it
reasonable to believe that another of those statements, the
conclusion, is true. (Thus, every argument claims that its premises
support its conclusion.)
5. Logic Strength: a property of arguments-----Logically strong/weak
An argument has logical strength when the premises, if true, actually
provide support for, justify, or make it reasonable to believe the
conclusion is true.
o Its idepedet of the truth or falsit of its preises. The fact that
either or both premises might be false does not affect the logical
strength of the argument. An argument with premises and
conclusion that are known to be true may be a very weak
argument.
o Features of Logical Strength
1) Feature 1
Logically strong: the premises, if true, support the truth of the
conclusion, or make it reasonable to believe the conclusion is
true. This does not mean the premises actually are true. It is
possible for a logically strong argument to have false premises.
2) Soundness: additional property of arguments and a means by
which we can assess them.
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