Briefing Document: Basic Logical Concepts - Deductive vs.

Inductive Arguments
Source: Excerpts from "Logic and CT- Week 3 (1).pdf" (Likely a textbook or course material
titled "Logic and Critical Thinking (2025)")
Date: October 26, 2023
Overview: This briefing document summarizes the fundamental concepts of deductive and
inductive arguments as presented in the provided source material. The main focus is on
distinguishing between these two types of arguments, understanding their respective
characteristics and claims, identifying common patterns of reasoning within each, and grasping
the concepts of deductive validity and inductive strength.
Main Themes and Important Ideas/Facts:
1. Distinction Between Deductive and Inductive Arguments:

• Deductive Arguments: These arguments aim to prove their conclusions with

rigorous, inescapable logical support. If the premises are true, the conclusion must
be true. The conclusion follows necessarily from the premises.
• Example: "All humans are mortal. Socrates is human. Therefore, Socrates is mortal."
• The source states: "Deductive arguments try to prove their conclusions with
rigorous, inescapable or airtight logical support for their conclusions."
• Key characteristics include:
• "If the premises are true, then the conclusion must be true."
• "The conclusion follows necessarily from the premises."
• "It is impossible for all the premises to be true and the conclusion false."
• "It is logically inconsistent to assert the premises and deny the conclusion; if you
accept the premises, you must accept the conclusion."
• Inductive Arguments: These arguments aim to show that their conclusions are
plausible or likely given the premise(s). Even if the premises are true, the conclusion
is only probably true. The conclusion follows probably from the premises.
• Example: "Polls show that 75 percent of NPP MPs favour the E-Levy
implementation. Ken Agyapong is NPP MP. Therefore, Ken Agyapong likely favours
the E-Levy implementation."
• The source states: "But Inductive arguments try to show that their conclusions are
plausible or likely given the premise(s)."
• Key characteristics include:
• "If the premises are true, then the conclusion is probably true."
• "The conclusion follows probably from the premises."
• "It is unlikely for the premises to be true and the conclusion false."
• "Although it is logically consistent to assert the premises and deny the conclusion,
the conclusion is probably true if the premises are true."

2. Determinants of Deductive and Inductive Arguments:

• Indicator Word Test: Certain words often signal whether an argument is intended
to be deductive or inductive.
 • Deductive Indicators: "certainly," "it logically follows that," "definitely," "it is
logical to conclude that," "absolutely," "this logically implies that," "conclusively,"
"this entails that."
• Inductive Indicators: "probably," "one would expect that," "likely," "it is a good bet
that," "it is plausible to suppose that," "chances are that," "it is reasonable to assume
that," "odds are that."
• Strict Necessity Test: This is a crucial determinant. If the conclusion must be true if
the premises are true (strict logical necessity), the argument is deductive. If not, it is
typically inductive.
• Examples:"Alan is a father. Therefore, Alan is a male." (Deductive - strict necessity)
• "Jill is a six-year-old girl. Therefore, Jill cannot run a mile in one minute flat."
(Deductive - strict necessity based on physical limitations)
• Common Pattern Test: Recognizing common argument structures can help
determine if an argument is deductive.
• Modus Ponens (Deductive): "If A then B. A. Therefore, B." Example: "If we're in
Paris, then we are in France. We are in Paris. Therefore, we are in France." The
source identifies this pattern and its Latin name, stating: "This is an argument
pattern that logicians call modus ponens, a Latin expression that means ‘affirmative
mode.’"
• Principle of Charity Test: When faced with a doubtful argument, we should
interpret it in the way most favorable to the arguer. This might involve considering
if the argument is stronger when viewed as inductive rather than deductive, or vice
versa.
• Example: The argument about Andy eating at a burnt-down restaurant is
presented. The source suggests: "...if we treat the argument as inductive, the
argument is a good inductive argument because the premises, if true, do make the
conclusion likely."

3. Common Patterns of Deductive Reasoning:

• Hypothetical Syllogism: A three-line argument with two premises and a conclusion

involving conditional ("if...then...") statements.
• Example: "If I want to keep my financial aid, I’d better study hard. I do want to keep
my financial aid. Therefore, I’d better study hard."
• Categorical Syllogism: A three-line argument where each statement begins with
"all," "some," or "no."
• Example: "All oaks are trees. All trees are plants. So, all oaks are plants." The source
defines it as: "Categorical Syllogism may be defined as a three-line argument in
which each statement begins with the word all, some, or so."
• Argument by Elimination: This reasoning seeks to rule out possibilities until only
one remains.
• Example: The scenario involving Dutch, Jack, and Celia as potential murderers,
where evidence eliminates Dutch and Jack, leading to the conclusion that Celia
committed the murder.
• Argument Based on Mathematics: The conclusion depends on a mathematical
calculation or measurement.
 • Example: "Eight is greater than four. Four is greater than two. Therefore, eight is
greater than two."
• Argument from Definition: The conclusion is true based on the definition of a key
word or phrase used in the argument.
• Examples: "Janelle is a cardiologist. Therefore, Janelle is a doctor." and "Bertha is an
aunt. It follows that she is a woman." The source explains this as: "...the conclusion is
presented as being “true by definition,” that is, as following simply by definition
from some key word or phrase used in the argument."

4. Common Patterns of Inductive Reasoning:

• Inductive Generalization: A statement that attributes a characteristic to all or most

members of a class, based on observations of some members.
• Examples: "All wild grizzly bears in the United States live west of the Mississippi
River," "Most college students work," "Men are so unromantic!" The source defines
it as: "A generalization...is a statement that attributes some characteristic to all or
most members of a given class."
• Predictive Argument: A prediction is supported with reasons.
• Example: "It has rained in Brong Ahafo every February since weather records have
been kept. Therefore, it will probably rain in Brong Ahafo next February." The
source notes: "Predictive arguments are among the most common patterns of
inductive reasoning."
• Argument from Authority: A claim is supported by citing a presumed authority or
witness.
• Example: "More Americans die of skin cancer each year than die in car accidents.
How do I know? My doctor told me."
• Causal Argument: Asserts or denies a causal relationship between two things.
• Examples: "I can’t log on. The network must be down." and "Rashid isn’t allergic to
peanuts. I saw him eat a bag of peanuts on the flight from Dallas."
• Statistical Argument: Rests on statistical evidence about a percentage of a group
having a certain characteristic.
• Examples: The argument about St. Stephen's students and Episcopalianism, and the
doctor's warning about the failure rate of condoms. The source states: "A statistical
argument rests on statistical evidence—that is, evidence that some percentage of
some group or class has some particular characteristic."
• Argument from Analogy: The conclusion depends on a comparison or similarity
between two or more things.
• Examples: The comparison between Hershey Park and Dorney Park's roller
coasters, and the inference about Paula's characteristics based on Bill and Mary's.
The source defines it as: "In an argument from analogy, the conclusion is claimed to
depend on an analogy (i.e., a comparison or similarity) between two or more
things."

5. Deductive Validity:
 • A deductive argument is valid if it is impossible for all the premises to be true and
the conclusion false.
• Key conditions for a valid argument:
• "If the premises are true, the conclusion must be true."
• "The conclusion follows necessarily from the premises."
• "The premises provide logically conclusive grounds for the truth of the conclusion."
• "It is logically inconsistent to assert all the premises as true and deny the
conclusion."
• Note that validity concerns the structure of the argument, not the truth of its
premises. A valid argument can have false premises and a false conclusion.

6. Inductive Strength:

• In a strong inductive argument, the conclusion follows probably from the premises.
• Key conditions for a strong inductive argument:
• "If the premises are true, the conclusion is probably true."
• "The premises provide probable, but not logically conclusive, grounds for the truth
of the conclusion."
• "The premises, if true, make the conclusion likely."
• Strength in inductive arguments is a matter of degree; an inductive argument can be
weak or strong depending on the support the premises provide for the conclusion.

Conclusion:
The provided source clearly outlines the fundamental differences between deductive and
inductive arguments, emphasizing their distinct aims, the nature of the support premises offer to
their conclusions, and the concepts of deductive validity and inductive strength. The inclusion of
indicator words, the strict necessity test, the principle of charity, and detailed descriptions with
examples of common deductive and inductive reasoning patterns provide a solid foundation for
understanding basic logical concepts in critical thinking.