Guide to Critical Thinking

Everyday Logic: Evidence, Beliefs, and Good Thinking
It is wrong always, everywhere, and for anyone to believe anything on insufficient evidence.
—W. K. Clifford (1879, p. 186)
British philosopher and mathematician W. K. Clifford’s claim—that it is unethical to believe 
anything if you do not have sufficient evidence for it—elicited a pronounced response from 
the philosophical community. Many argued that Clifford’s claim was too strong and that it 
is acceptable to believe things for which we lack the requisite evidence. Whether or not one 
absolutely agrees with Clifford, he raises a good point. Every day, millions of people make deci-
sions based on insufficient evidence. They claim that things are true or false without putting in 
the time, effort, and research necessary to make those claims with justification.
You have probably witnessed an argument in which people continue to make the same claims 
until they either begin to become upset or merely continue to restate their positions without 
adding anything new to the discussion. These situations often devolve and end with state-
ments such as, “Well, I guess we will just agree to disagree” or “You are entitled to your opin-
ion, and I am entitled to mine, and we will just have to leave it at that.” However, upon further 
reflection we have to ask ourselves, “Are people really entitled to have any opinion they want?”
(continued)
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Section 1.1 
What Is Critical Thinking?
From the perspective of critical thinking, the answer is no. Although people are legally entitled 
to their beliefs and opinions, it would be intellectually irresponsible of them to feel entitled to 
an opinion that is unsupported by logical reasoning and evidence; people making this claim 
are conflating freedom of speech with freedom of opinion. A simple example will illustrate this 
point. Suppose someone believes that the moon is composed of green cheese. Although he is 
legally entitled to his belief that the moon is made of green cheese, he is not rationally entitled 
to that belief, since there are many reasons to believe and much evidence to show that the 
moon is not composed of green cheese.
Good thinkers constantly question their beliefs and examine multiple sources of evidence to 
ensure their beliefs are true. Of course, people often hold beliefs that seem warranted but are 
later found not to be true, such as that the earth is flat, that it is acceptable to paint baby cribs 
with lead paint, and so on. However, a good thinker is one who is willing to change his or her 
views when those views are proved to be false. There are certain criteria that must be met for 
us to claim that someone is entitled to a specific opinion or position on an issue.
There are other examples where the distinction is not so clear. For instance, some people believe 
that women should be subservient to men. They hold this belief for many reasons, but the pre-
dominant one is because specific religions claim this is the case. Does the fact that a religious text 
claims that women should serve men provide sufficient evidence for one to believe this claim? 
Many people believe it does not. However, many who interpret their religious texts in this man-
ner would claim that these texts do provide sufficient evidence for such claims.
It is here that we see the danger and difficulty of providing hard-and-fast definitions of what 
constitutes sufficient evidence. If we believe that written words in books came directly from 
divine sources, then we would be prone to give those words the highest credibility in terms of 
the strength of their evidence. However, if we view written words as arguments presented by 
their authors, then we would analyze the text based on the evidence and reasoning presented. 
In the latter case we would find that these people are wrong and that they are merely making 
claims based on their cultural, male-dominated environments.
Of course, all people have the freedom to believe what they want. However, if we think of 
entitlement as justification, then we cannot say that all people are entitled to their opinions 
and beliefs. As you read this book, think about what you believe and why. If you do not have 
reasons or supporting evidence for your beliefs and opinions, you should attempt to find it. 
Try not to get sucked into arguments without having evidence. Most important, as a good 
thinker, you should be willing and able to admit the strengths and weaknesses of various posi-
tions on issues, especially your own. At the same time, if in your search for evidence you find 
that the opposing position is the stronger one, you should be willing to change your position. 
It is also a sign of good thinking to suspend judgment when you suspect that the arguments of 
others are not supported by evidence or logical reasoning. Suspending judgment can protect 
you from error and making rash decisions that lead to negative outcomes.