Jonathan Dolhenty, Ph.D. Logic and Critical Thinking.
TRUTH AND THINKING
Truth is the object of thinking.
Some truths are obvious; others are difficult to
Some judgments we make are simple; some
judgments are complicated.
Some arguments, whether made by us or others,
may be straightforward and easily understood;
other arguments may be complex and consist of a
series of smaller arguments, each needing to be
critically examined and evaluated.
CRITICAL THINKING AND LOGIC
Every object of knowledge has a branch of knowledge which studies it.
Planets, stars, and galaxies are studied by astronomy.
Chemistry studies the structure, composition, and properties of material
substances and the transformations they undergo.
The origin, evolution, and development of human society is the object studied by
Economics, biology, geography, and grammar all have objects of knowledge
which they investigate, describe, and try to explain.
WHAT ABOUT CRITICAL?
Critical thinking involves knowledge of the science of logic, including the skills of
logical analysis, correct reasoning, and understanding statistical methods.
Critical thinking, however, involves more than just an understanding of logical
A good critical thinker must also understand the sources of knowledge, the nature of
knowledge, and the nature of truth.
THE SCIENCE OF LOGIC
The object of knowledge involved in the science
of logic is "thinking," but it is "thinking"
approached in a special way.
Generally speaking, logic is that branch of
knowledge which reflects upon the nature of
But this may confuse logic with other branches of
knowledge which also have the nature of
"thinking" as a part of their specific object of
“THINKING” AND LOGIC
Logic doesn't just deal with "thinking" in general. Logic
deals with "correct thinking."
Training in logic should enable us to develop the skills
necessary to think correctly, that is, logically.
A very simple definition would be:
Logic is the subject which teaches you the rules for
correct and proper reasoning.
A more complete and "sophisticated" definition of logic, you
can define it this way:
Logic is the science of those principles, laws, and
methods, which the mind of man in its thinking must
follow for the accurate and secure attainment of truth.
A KIND OF LOGIC: NATURAL
“Natural Logic" or Common Sense
We all have an internal sense of what is
logical and what is not, which we generally
refer to as "common sense."
This "natural" logic we have learned from
the moment of birth, through our personal
experiences in the world and through our
acquisition of language.
A KIND OF LOGIC: SCIENTIFIC
Scientific logic is simply our natural
logic trained and developed to
expertness by means of well-
established knowledge of the principles,
laws, and methods which underlie the
various operations of the mind in the
pursuit of and attainment of truth.
LOGIC AS A SCIENCE
AND AN ART
Logic as a science:
The science part is the knowledge of the principles, laws, and methods of
Logic as an art:
Logic must be put into action or else the knowledge provided within the
science of logic is of little use.
We can speak of the "art" of logic, that is, the practical application of the
science of logic to our everyday affairs.
Logic as a science and an art
Logic is not intended merely to inform or instruct.
It is also a directive and aims at assisting us in the proper use of our power
In this sense, we can speak of logic as both a science and an art, a
practical art meant to be applied in our ordinary affairs.
Why Study Logic?
Aim: To develop a system of methods and
principles that we may use as criteria for
evaluating the arguments of others and as
guides in constructing arguments of our
Benefits: an increase in confidence that we
are making sense when we criticize the
arguments of others and when we advance
arguments of our own.
THREE OPERATIONS OF THE
OPERATIONS PRODUCT EXTERNAL
OF THE MIND SIGNS
Simple Concept / Idea Oral / written
Judgment Mental Oral / Written
Proposition Proposition or
Reasoning Mental Oral / written
agreement / argument
Simple Apprehension, Judgment,
Simple Apprehension: an operation of the mind
whereby we abstract from the non-essential elements
of a thing and recognize those essential elements
which make it to be precisely that particular thing.
Judgment: an operation of the mind which unites two
ideas by affirmation or separates by negation.
Reasoning / Mediate Inference: an operation of the
mind that involves a process whereby from certain
truths already known, we proceed to another which is
different from those that are given but necessarily
following from them.
Basic Concept: ARGUMENTS
Argument: a group of statements, one of
which (the conclusion) is claimed to follow
from the other or others (the premises).
Good arguments: those in which the
conclusion really does follow from the
Bad arguments: those in which does not,
even though it is claimed to
Basic Concept: Statement
Basis: Argument as a group of statement
Statement: a sentence that is either true or
false; typically a declarative sentence.
1. Hydrogen is combustible.
2. World War II began in 1939.
3. Some ducks are fish.
4. Abraham Lincoln was beheaded.
Basic Concept: Truth - Value
Truth value of the statement: the attribute
by which a statement is either true or false.
1. Hydrogen is combustible. (true)
2. World War II began in 1939. (true)
3. Some ducks are fish. (false)
4. Abraham Lincoln was beheaded. (false)
Basic Concept: Non-Statements
Sentences which cannot be said to be either
true or false.
1. What is the atomic weight of carbon? (question)
2. Let’s go to the park today. (proposal)
3. We suggest that you travel by bus. (suggestion)
4. Turn to the left at the next corner. (command)
5. Ouch! (exclamation)
Components of an Argument:
Premise(s) and conclusion
Premises: the statement that set forth the evidence.
Conclusion: the statement that is claimed to follow from
All cats are animals.
Felix is a cat.
Therefore, Felix is an animal.
N.B. the first two statements are the premises; the third
is the conclusion.
The claim that the conclusion follows from the premises
is indicated by the word “therefore”.
Schema of an Argument:
Premises Statement Evidence
Conclusion Statement What is
One of the most important tasks in the
analysis of arguments is being able to
distinguish premises from conclusion.
If what is thought to be a conclusion is really a
premise, and vice versa, the subsequent analysis
cannot possibly be correct.
Frequently, arguments contain certain indicator
words that provide clues in identifying premises
A word that provides a clue to identifying a conclusion.
Therefore hence whence
Wherefore thus consequently
Accordingly so it follows that
Entails that as a result We may conclude
Implies that it must be that We may infer
Whenever a statement follows one of these indicators, it can
usually be identified as the conclusion.
By process of elimination the other statements in the
argument are the premises.
Example: This is pen is out of ink. Consequently, it will not
A word that provides a clue to identifying a premise.
If an argument does not contain a conclusion indicator, it may
contain a premise indicator.
for the reason that in that seeing that
As indicate by for since
Because as inasmuch as
may be inferred from given that owing to
Any statement following one of these indicators can usually
be identified as a premise.
Example: This locket is worth a lot of money, since it is made
Basic Concept: Inference &
An inference, in the technical sense of the
term, is the reasoning process used to
produce an argument.
A proposition, in the technical sense, is the
meaning or information content of a
Passages lacking an inferential claim
Passages lacking an inferential claim contain statements that could
be premises or conclusions (or both) but what is missing is a claim
that a reasoning process is being expressed.
Warnings/ pieces of advice: kinds of discourse aimed at
modifying someone’s behavior.
Ex. “Watch out that you don’t slip on the floor.”
Ex. “I suggest you take philosophy in the first semester.”
Each of these could serve as the conclusion of an argument; but in their
present context, there is no claim that they supported or implied by reasons
Statements of beliefs or opinion: expressions of what someone
happen to believe or think at a certain time.
Ex. I think a nation such as ours, with its high moral traditions and
commitments, has a further responsibility to know how to became drawn
into this conflict, and to learn the lessons it has to teach us for the future.
Alfred Hassler, Saigon, U.S,A.
Passages lacking an inferential claim
Loosely associated statements: may be about the same general subject,
but they lack a claim that one of them is proved by the others.
Ex. Not to honor men of worth will keep the people from contention; not
to value goods that are hard to come by will keep them from theft; not
to display what is desirable will keep them from being unsettled of
Lao-Tzu, Thoughts from the Tao Te Ching
Report: consists of a group of statements that convey information about
some situation or event. Ex. News Report
Expository passage: a kind of discourse that begins with a topic sentence
followed by one or more sentences that develop the topic sentence.
Illustration: consists of a statement about certain subject combined with a
reference to one or more specific instances intended to exemplify that
Ex. Chemical elements, as well as compounds, can be represented by
molecular formulas. Thus, oxygen is represented by “O 2”, sodium
chloride by “NaCl”, and sulfuric acid by “H 2SO4”.
A conditional statement is an “if… then…” statement.
Ex. If it rains, the soil is wet.
It is made up of two component statements: if = antecedent;
then = consequent
Occasionally, “then” is left out
Conditional statements are not arguments because there is no
claim that either the antecedent or the consequent presents
In other words, there is no assertion that either the antecedent or
the consequent is true. Rather, there is only the assertion that if the
antecedent is true, then so is the consequent.
A conditional statement may serve as premise or the
conclusion of an argument.
An explanation consists of a statement or a group of statements
intended to shed light on some phenomenon that is usually
accepted as a matter of fact.
Ex. Cows can digest grass, while humans cannot, because their
digestive systems contain enzymes not found in humans.
Explanandum: the statement that describes the event or
phenomenon to be explained.
Explanans: the statement or group of statements that purports
to do the explaining.
Explanations are sometimes mistaken for arguments because they
often contain the indicator word “because”. Yet explanations are not
arguments for the following reason: In an explanation, the
explanans is intended to show why something is the case, whereas
in an argument the premises are intended to prove that something
is the case.
Premises facts Explanans
shed light on