Constructive Dilemma
A valid argument form that uses a disjunctive first premise and two conditional premises to conclude a disjunction of the consequents. It is useful for reasoning about choices and their outcomes.
THE IDEA
CONSTRUCTIVE DILEMMA
A constructive dilemma combines a disjunction with conditional statements about each alternative. If the premises hold, at least one of the resulting consequences must hold. Here is an example:
We will either rent or buy a home. If we rent, then we will need to move. If we buy, then we will stay. So, we will either move or we will stay.
The symbolic form of this constructive dilemma is as follows:
Either A or B. If A then C. If B then D. So, either C or D.
Here we see that the constructive dilemma begins with a disjunctive statement as its first premise. In the second and third premises, it contains conditional statements. Finally, it concludes with a disjunctive statement that affirms the consequents of the second and third premises. Here are some more examples of the constructive dilemma:
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Either we celebrate Hanukkah or we celebrate Christmas. If we celebrate Hanukkah, then we will drive to Los Angeles. If we celebrate Christmas, then we will drive to San Francisco. Therefore, we will either drive to Los Angeles or to San Francisco.
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Humans either have instrumental value or intrinsic value. If humans have instrumental value, then their worth is measured by their efficiency. If humans have intrinsic value, then they have worth regardless of how efficient they are. So, either human worth is measured by efficiency or humans have worth regardless of their efficiency.
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Either Sigmund Freud was correct or C.S. Lewis was correct. If Sigmund Freud was correct, then the idea of God is simply a projection of unconscious desires. If C.S. Lewis was correct, then the theism of the Bible is true, and God exists independently of our unconscious desires. So, either God is a projection of our unconscious desires, or God exists independently of our unconscious desires.
The form is valid, but the premises still need support. For example, the claim that either Freud or Lewis must be correct needs its own defense; the symbolic pattern does not establish it. The alternatives also need not be mutually exclusive. Check both the proposed alternatives and the conditional claims before relying on the conclusion.
When to use it
Planning: State the alternatives and what would follow from each, then check whether the premises fit the actual decision.
Debate: Structure an argument to show that regardless of which of two opposing premises is true, a certain disjunction of outcomes follows.
Decision analysis: Evaluate a binary choice by explicitly stating the conditional outcomes and concluding what must happen in either case.
Problem solving: Break down a problem with two alternative paths into a logical form that clarifies the necessary results.
FROM THEORY TO EVERYDAY LIFE
In practice
Renting vs. Buying a Home
We will either rent or buy a home. If we rent, then we will need to move. If we buy, then we will stay. So, we will either move or we will stay.
Holiday Celebration and Travel
Either we celebrate Hanukkah or we celebrate Christmas. If we celebrate Hanukkah, then we will drive to Los Angeles. If we celebrate Christmas, then we will drive to San Francisco. Therefore, we will either drive to Los Angeles or to San Francisco.
Human Value Theories
Humans either have instrumental value or intrinsic value. If humans have instrumental value, then their worth is measured by their efficiency. If humans have intrinsic value, then they have worth regardless of how efficient they are. So, either human worth is measured by efficiency or humans have worth regardless of their efficiency.
Freud vs. Lewis on God
Either Sigmund Freud was correct or C.S. Lewis was correct. If Freud was correct, then God is a projection of unconscious desires. If Lewis was correct, then God exists independently. So, either God is a projection or God exists independently.
A MOMENT TO REFLECT
Put it to work.
Think of a decision you face. Can you state alternatives that cover the possibilities under consideration and give a supported conditional consequence for each?
Why might the constructive dilemma be considered 'valid' even if its premises are false? How does that affect its use in argumentation?
How does the constructive dilemma differ from a simple conditional argument like modus ponens? What does it add to reasoning?
CONTINUE THE CONVERSATION
Source texts
- Informal Logical Fallacies A Brief Guide