2.5: Deductive Reasoning
What if, in a fictitious far-away land, a poor peasant were awaiting his fate from the king? He is standing in a stadium, filled with spectators pointing and wondering what is going to happen. Finally, the king directs everyone’s attention to two doors, at the floor level with the peasant. Both doors have signs on them, which are below:
Door A | Door B |
---|---|
IN THIS ROOM THERE IS A LADY, AND IN THE OTHER ROOM THERE IS A TIGER. | IN ONE OF THESE ROOMS THERE IS A LADY, AND IN ONE OF THE OTHER ROOMS THERE IS A TIGER. |
The king states, “Only one of these statements is true. If you pick correctly, you will find the lady. If not, the tiger will be waiting for you.” Which door should the peasant pick? After completing this Concept, you'll be able to use deductive reasoning to solve this problem.
Watch This
CK-12 Foundation: Chapter2DeductiveReasoningA
James Sousa: Introduction to Deductive Reasoning
Guidance
Logic is the study of reasoning. Deductive reasoning draws conclusions from facts. Typically, conclusions are drawn from general statements about something more specific.
Law of Detachment
Here are two true statements:
- Every odd number is the sum of an even and an odd number.
- 5 is an odd number.
What can you conclude? Based on only these two true statements, there is one conclusion: 5 is the sum of an even and an odd number. (This is true, \begin{align*}5 = 3 + 2\end{align*}
\begin{align*}p: \ \text{A number is odd} \qquad q: \ \text{It is the sum of an even and odd number}\end{align*}
So, the first statement is \begin{align*}p \rightarrow q\end{align*}
\begin{align*}& p \rightarrow q\\
& p\\
& \therefore q \qquad \qquad \therefore \ \text{symbol for “therefore”}\end{align*}
All deductive arguments that follow this pattern have a special name, the Law of Detachment. The Law of Detachment says that if \begin{align*}p \rightarrow q\end{align*} is a true statement and given \begin{align*}p\end{align*}, then you can conclude \begin{align*}q\end{align*}. Another way to say the Law of Detachment is: “If \begin{align*}p \rightarrow q\end{align*} is true, and \begin{align*}p\end{align*} is true, then \begin{align*}q\end{align*} is true.”
Law of Contrapositive
The following two statements are true:
- If a student is in Geometry, then he or she has passed Algebra I.
- Daniel has not passed Algebra I.
What can you conclude from these two statements? These statements are in the form:
\begin{align*}& p \rightarrow q\\ & \sim q\end{align*}
\begin{align*}\sim q\end{align*} is the beginning of the contrapositive \begin{align*}(\sim q \rightarrow \sim p)\end{align*}, therefore the logical conclusion is \begin{align*}\sim p\end{align*}: Daniel is not in Geometry. This example is called the Law of Contrapositive. The Law of Contrapositive says that if \begin{align*}p \rightarrow q\end{align*} is a true statement and given \begin{align*}\sim q\end{align*}, then you can conclude \begin{align*}\sim p\end{align*}. Recall that the logical equivalent to a conditional statement is its contrapositive. Therefore, the Law of Contrapositive is a logical argument.
Law of Syllogism
Determine the conclusion from the following true statements.
- If Pete is late, Mark will be late.
- If Mark is late, Karl will be late.
So, if Pete is late, what will happen? If Pete is late, this starts a domino effect of lateness. Mark will be late and Karl will be late too. So, if Pete is late, then Karl will be late, is the logical conclusion. Each “then” becomes the next “if” in a chain of statements. The chain can consist of any number of connected statements. This is called the Law of Syllogism. The Law of Syllogism says that if \begin{align*}p \rightarrow q\end{align*} and \begin{align*}q \rightarrow r\end{align*} are true, then \begin{align*}p \rightarrow r\end{align*} is the logical conclusion.
Typically, when there are more than two linked statements, we continue to use the next letter(s) in the alphabet to represent the next statement(s); \begin{align*}r \rightarrow s, s \rightarrow t\end{align*}, and so on.
Example A
Suppose Bea makes the following statements, which are known to be true.
If Central High School wins today, they will go to the regional tournament.
Central High School won today.
What is the logical conclusion?
This is an example of deductive reasoning. There is one logical conclusion if these two statements are true: Central High School will go to the regional tournament.
Example B
Here are two true statements. Be careful!
If \begin{align*}\angle A\end{align*} and \begin{align*}\angle B\end{align*} are a linear pair, then \begin{align*}m \angle A + m \angle B = 180^\circ\end{align*}.
\begin{align*}m \angle 1 = 90^\circ\end{align*} and \begin{align*}m \angle 2 = 90^\circ\end{align*}.
What conclusion can you draw from these two statements?
Here there is NO conclusion. These statements are in the form:
\begin{align*}& p \rightarrow q\\ & q\end{align*}
We cannot conclude that \begin{align*}\angle 1\end{align*} and \begin{align*}\angle 2\end{align*} are a linear pair. We are told that \begin{align*}m \angle 1 = 90^\circ\end{align*} and \begin{align*}m \angle 2 = 90^\circ\end{align*} and while \begin{align*}90^\circ + 90^\circ = 180^\circ\end{align*}, this does not mean they are a linear pair. Here are two counterexamples.
In both of these counterexamples, \begin{align*}\angle 1\end{align*} and \begin{align*}\angle 2\end{align*} are right angles. In the first, they are vertical angles and in the second, they are two angles in a rectangle.
This is called the Converse Error because the second statement is the conclusion of the first, like the converse of a statement.
Example C
Determine the conclusion from the true statements below.
Babies wear diapers.
My little brother does not wear diapers.
The second statement is the equivalent of \begin{align*}\sim q\end{align*}. Therefore, the conclusion is \begin{align*}\sim p\end{align*}, or: My little brother is not a baby.
Watch this video for help with the Examples above.
CK-12 Foundation: Chapter2DeductiveReasoningB
Concept Problem Revisited
Analyze the two statements on the doors.
Door A: IN THIS ROOM THERE IS A LADY, AND IN THE OTHER ROOM THERE IS A TIGER.
Door B: IN ONE OF THESE ROOMS THERE IS A LADY, AND IN ONE OF THE OTHER ROOMS THERE IS A TIGER.
We know that one door is true, so the other one must be false. Let’s assume that Door A is true. That means the lady is behind Door A and the tiger is behind Door B. However, if we read Door B carefully, it says “in one of these rooms,” which means the lady could be behind either door, which is actually the true statement. So, because Door B is the true statement, Door A is false and the tiger is actually behind it. Therefore, the peasant should pick Door B.
Vocabulary
Logic is the study of reasoning. Deductive reasoning draws conclusions from facts.
Guided Practice
1. Here are two true statements.
If \begin{align*}\angle A\end{align*} and \begin{align*}\angle B\end{align*} are a linear pair, then \begin{align*}m \angle A + m \angle B = 180^\circ\end{align*}.
\begin{align*}\angle ABC\end{align*} and \begin{align*}\angle CBD\end{align*} are a linear pair.
What conclusion can you draw from this?
2. Determine the conclusion from the true statements below.
If you are not in Chicago, then you can’t be on the \begin{align*}L\end{align*}.
Bill is in Chicago.
3. Determine the conclusion from the true statements below.
If you are not in Chicago, then you can’t be on the \begin{align*}L\end{align*}.
Sally is on the \begin{align*}L\end{align*}.
Answers:
1. This is an example of the Law of Detachment, therefore:
\begin{align*}m \angle ABC + m \angle CBD = 180^\circ\end{align*}
2. If we were to rewrite this symbolically, it would look like:
\begin{align*}& \sim p \rightarrow \sim q\\ & p\end{align*}
This is not in the form of the Law of Contrapositive or the Law of Detachment, so there is no logical conclusion. You cannot conclude that Bill is on the \begin{align*}L\end{align*} because he could be anywhere in Chicago. This is an example of the Inverse Error because the second statement is the negation of the hypothesis, like the beginning of the inverse of a statement.
3. If we were to rewrite this symbolically, it would look like:
\begin{align*}& \sim p \rightarrow \sim q\\ & q\end{align*} Even though it looks a little different, this is an example of the Law of Contrapositive. Therefore, the logical conclusion is: Sally is in Chicago.
Practice
Determine the logical conclusion and state which law you used (Law of Detachment, Law of Contrapositive, or Law of Syllogism). If no conclusion can be drawn, write “no conclusion.”
- People who vote for Jane Wannabe are smart people. I voted for Jane Wannabe.
- If Rae is the driver today then Maria is the driver tomorrow. Ann is the driver today.
- If a shape is a circle, then it never ends. If it never ends, then it never starts. If it never starts, then it doesn’t exist. If it doesn’t exist, then we don’t need to study it.
- If you text while driving, then you are unsafe. You are a safe driver.
- If you wear sunglasses, then it is sunny outside. You are wearing sunglasses.
- If you wear sunglasses, then it is sunny outside. It is cloudy.
- I will clean my room if my mom asks me to. I am not cleaning my room.
- If I go to the park, I bring my dog. If I bring my dog, we play fetch with a stick. If we play fetch, my dog gets dirty. If my dog gets dirty, I give him a bath.
- Write the symbolic representation of #3. Include your conclusion. Is this a sound argument? Does it make sense?
- Write the symbolic representation of #1. Include your conclusion.
- Write the symbolic representation of #7. Include your conclusion.
For questions 12 and 13, rearrange the order of the statements (you may need to use the Law of Contrapositive too) to discover the logical conclusion.
- If I shop, then I will buy shoes. If I don’t shop, then I didn’t go to the mall. If I need a new watch battery, then I go to the mall.
- If Anna’s parents don’t buy her ice cream, then she didn’t get an \begin{align*}A\end{align*} on her test. If Anna’s teacher gives notes, Anna writes them down. If Anna didn’t get an \begin{align*}A\end{align*} on her test, then she couldn’t do the homework. If Anna writes down the notes, she can do the homework.
Determine if the problems below represent inductive or deductive reasoning. Briefly explain your answer.
- John is watching the weather. As the day goes on it gets more and more cloudy and cold. He concludes that it is going to rain.
- Beth’s 2-year-old sister only eats hot dogs, blueberries and yogurt. Beth decides to give her sister some yogurt because she is hungry.
- Nolan Ryan has the most strikeouts of any pitcher in Major League Baseball. Jeff debates that he is the best pitcher of all-time for this reason.
- Ocean currents and waves are dictated by the weather and the phase of the moon. Surfers use this information to determine when it is a good time to hit the water.
- As Rich is driving along the 405, he notices that as he gets closer to LAX the traffic slows down. As he passes it, it speeds back up. He concludes that anytime he drives past an airport, the traffic will slow down.
- Amani notices that the milk was left out on the counter. Amani remembers that she put it away after breakfast so it couldn’t be her who left it out. She also remembers hearing her mother tell her brother on several occasions to put the milk back in the refrigerator. She concludes that he must have left it out.
- At a crime scene, the DNA of four suspects is found to be present. However, three of them have an alibi for the time of the crime. The detectives conclude that the fourth possible suspect must have committed the crime.
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Image Attributions
Here you'll learn how to deductively draw conclusions from facts using the Law of Detachment, the Law of Contrapositive, and the Law of Syllogism.