Media, Information and Technoculture 1050A/B Chapter Notes - Chapter 3: Deductive Reasoning, Validity
23 Jul 2018
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Chapter 3 Notes
• Defined an argument as a group of statements in which some of them (the premises)
are intended to support another of them (conclusion)
• To recognize an argument, you must be able to identify the premises and the
conclusion. Indicator words such as because and since often signal the presence of
premises, and words such as therefore and thus can point to a conclusion
Argument Basics
• The point of devising an argument is to try to show that a statement or claim is worthy
of acceptance
• The point of evaluating an argument is to see whether this task has been successful –
whether the argument shows that the statement (the conclusion) really is worthy of
acceptance
• When the argument shows that the statement is worthy of acceptance, we say that the
argument is good
• Arguments come in two forms – deductive and inductive
o Deductive arguments are intended to provide logically conclusive support for its
conclusion
o Inductive arguments are intended to provide probable – not conclusive –
support for its conclusion
• A deductive argument that succeeds in providing such decisive logical support is said to
be valid
o A deductively valid argument is such that if its premises are true, its conclusion
must be true (if the premises are true, there is no way that the conclusion can be
false)
o *Valid is not a synonym for true, valid = logical structure that guarantees the
truth of the conclusion if the premises are true
▪ The conclusion follows logically from the premises
o Because of the guarantee of truth in the conclusion, deductively valid arguments
are said to be truth-preserving
• A deductive argument that fails to provide such support is said to be invalid
o In a deductively invalid argument, the conclusion does not follow logically from
the premises (even if the premises were true, they would not guarantee that
their conclusions are true)
o The conclusion doesn’t follow from the premises offered
• *Even an argument with a conclusion that is obviously true can be invalid if that
conclusion is not supported by the premises offered
• An inductive argument that succeeds in providing probable – but not conclusive – logical
support for its conclusion is said to be strong
o An inductively strong argument is such that if its premises are true, its conclusion
is probably or likely to be true
o The structure of an inductively strong argument cannot guarantee that the
conclusion is true if the premises are true, but the conclusion can be rendered
probably and worthy of acceptance
• An inductive argument that fails to provide such support is said to be weak
• Inductive arguments are not truth-preserving
• Logical validity or logical strength is an essential characteristic of good arguments
• Good arguments also have true premises
• A good argument is one that has the proper structure and true premises
• A deductively valid argument that has true premises is said to be sound
o A sound argument is a good argument that gives you reasons for accepting its
conclusion
• Deductively valid arguments can have true premises (sound) or false premises (not
sound) and true or false conclusions
o False premises, false conclusion
o False premises, true conclusion (kind of by accident)
o True premises, true conclusion
• When inductively strong arguments have true premises, they are said to be cogent
• Deductive arguments give a conclusion that is absolute – either the conclusion is shown
to be true, or it is not
Judging Arguments
• When you come face to face with an argument to evaluate, (1) how can you tell
whether it’s deductive or inductive, and (2) how can you determine whether it gives you
good reasons for accepting the conclusion (whether it’s sound or cogent)?
Steps:
1. Find the conclusion of the argument and then its premises (such as indicator words)
2. Ask, “is it the case that if the premises are true the conclusion must be true?”
o If yes, the argument is deductive – for it is very likely meant to offer conclusive
support for its conclusion
o The argument then is deductively valid, and you then check to see if it’s sound
o If no, go to step 3.
3. Ask, “is it the case that if the premises are true its conclusion is probably true?”
Document Summary
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