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What Is Deductive Reasoning?

Deductive reasoning is the process by which something is determined, based on pre-existing and accepted facts (or premises). When using deductive reasoning, a person selects the single best outcome or solution to a problem, supporting their choice with knowledge that already exists and has been determined to be true. Deductive reasoning is used in all sorts of situations, from day-to-day decisions in contemporary life to large-scale ones like those in policymaking or governmental settings.

Deductive Reasoning Components

Deductive reasoning is dependent on a set of pre-existing premises. Premises are the accepted facts, or knowledge base, that inform decision making, particularly in the context of deductive reasoning. Premises are not tested every time they are used; they are accepted as truth. Based on specific premises, conclusions can be drawn via deductive reasoning. For example, if it is cold outside, people will not be wearing shorts today. Thus, if Max goes outside, it can be deduced that Max will not be wearing shorts.

It is also important to note, however, that there can always be a counter arguments to claims made via deductive reasoning. Arguments against the claim that Max will not be wearing shorts because it is cold outside, for example, might look like, “how cold is cold? Doesn’t everyone experience temperatures differently?,” or “Max could be going for a run, and wearing shorts for a run outside.” Deductive reason’s dependence on knowledge premises leaves room for debate, because those premises are accepted truths that are not experimented on, or examined for veracity, every time they are used in decision making.

Deductive Reasoning Examples

Examples of deductive reasoning in everyday situations:

  • Milo has no cake flour. He needs cake flour in order to bake brownies. He cannot make brownies.
    • Based on the premises that cake flour is needed to bake brownies and Milo does not have any, deductive reasoning determines that Milo cannot make brownies. The accepted facts (lack of flour, need for flour) inform the decision made.
  • Layla wants to bike to and from school. The weather report says that there will be heavy rain in the afternoon that will make the roads unsafe. Layla cannot safely bike to and from school.
    • Layla may be able to bike to school safely, but she will not be able to do so in the afternoon due to the rain. Based on these premises, she cannot bike safely.
  • Paolo needs to finish a math project tonight. Paolo also has a basketball game tonight. If Paolo turns in the project late, he will get a lower grade. If he misses the game, he will be removed from the team. Paolo will not finish his math project.
    • Paolo needs to finish a project, but needs to also stay on his basketball team. The stakes, as presented in the premises, are higher for basketball than the math project. Paolo will forgo the latter in order to succeed with the former.

Deductive Reasoning in Other Fields

Deductive reasonings can be seen in several different fields.

  • Example of deductive reasoning in chemistry: Opposites attract; negative and positive molecules attract one another. Two molecules force each other away instead of being attracted to one another. The two molecules are both negative or both positive.
  • Example of deductive reasoning in biology: Animals that nurse their young are mammals. Mammals are warmblooded. Warmblooded animals must nurse their young.
  • Example of deductive reasoning in math: In the order of operations, multiplication is done before addition is. Addition is done before subtraction is. Multiplication must be done before subtraction is.

Deductive Reasoning Uses

Deductive reasoning is used in the workplace for problem solving, as well as when organizing daily tasks and workflow. When problem solving, deductive reasoning allows co-workers to prioritize tasks or identify a solution as quickly as possible. For example, if it is an accepted fact on a team of editors that a manuscript is due on Friday, both the writers and illustrators are experiencing delays, but the writers always meet their deadlines without outside pressure, then the editorial team knows that they need to focus on supporting the illustrators.

When daily tasks and workflow are being laid out for a workplace, deductive reasoning can help streamline the process and ensure that tasks are completed as efficiently as possible. For example, if the project manager knows that the Spanish writing team is bigger than the English writing team and has more experience with writing science books for third graders, the project manager can expect the Spanish team to finish their book-writing before the English team.

Deductive reasoning supports workplace planning and efficiency.


Deductive Reasoning Types

There are two main types of deductive reasoning: syllogism and conditional reasoning. Syllogism occurs when conclusions are drawn from two premises where both premises share a term with the conclusion. (e.g.: If X is equal to Y, and Z is equal to X, then Z is equal to Y). The use of syllogism aids in the effort to present an incontestable argument. While deductive reasoning depends on premises that are accepted as truth, utilizing a syllogism then strengthens that claim by demonstrating the inevitability of its outcome.

Conditional reasoning, on the other hand, uses “if…then” statements to make an argument grounded in deductive reasoning. In this context, an antecedent (the “if”) is in relationship with a consequent (the “then”). In this context, an argument is valid when all of its components parts are true, and their outcome must also be true. An argument is invalid when its component parts are true, but the outcome can be false. If there is any chance of the outcome being false, then the argument is invalid.

Deductive reasoning uses deductive logic to determine a solution or outcome based on existing premises. Logic is defined as a linear, step-by-step means of thinking about, or solving, something.

There are many different types of deductive arguments that one might use to deduce the best solution to a situation, or to counter-argue another’s logical reasoning. The examples are as follows:

  • Affirming the Antecedent (Valid): If X is true, then Y is true. Thus, X is true, therefore Y is true.
  • Affirming the Consequent (Invalid): If X is true, then Y is true. But if Y is true, it does NOT guarantee that X is true.
  • Denying the Antecedent (Invalid): If X is true, then Y is true. But if X is NOT true, it does NOT guarantee that Y is not true.
  • Denying the Consequent (Valid): If X is true, then Y is true. If Y is not true, then X is not true.
  • Addition: If X and Y mean the same thing, they can be used interchangeably.
  • Simplification: If X and Y mean the same thing, one can choose to only use X.
  • Dilemma: If one must choose between X and Y, but X equals Y AND X equals Z, one chooses Z.
  • Contraposition: If X occurs, then Y occurs. If X does not occur, then Y does not occur.
  • Hypothetical Syllogism: If X occurs, then Y occurs. If Y occurs, then Z occurs. Thus, if X occurs then Z occurs.
  • Disjunctive Syllogism: If one must choose between X and Y, and one choose not-X, then one has chosen Y.
  • Modus Ponens: If X occurs, then Y occurs.
  • Modus Tollens: If X occurs then Y occurs. If Y has not occurred, then X did not occur.

If red is valued the same as yellow, and green is valued the same as yellow, then red and green have the same value.

Logic graphic

Deductive Reasoning Versus Inductive Reasoning

Deductive reasoning differs from inductive reasoning in that deductive reasoning promises a truthful argument, so long as the truth of its premises remain intact. Inductive reasoning seeks to make an argument that likely will be true if its premises are true.

Deductive reasoning: Cats ruin couches. Couches are expensive. Cats ruin expensive furniture.

Inductive reasoning: Every time I’ve owned a cat, it has ruined my couch. Couches are expensive. So if I get another cat, it will ruin my expensive furniture.

Benefits of Deductive Reasoning

  • Focuses on one solution
  • Almost guarantees a true conclusion
  • Aids in the decision making process
  • Shows the step-by-step process that informs a decision.

Lesson Summary

Deductive reasoning is a form of logical thinking that helps people make claims and conduct arguments that have high odds of being true, provided that the premises upon which they are grounded are also true. Where inductive reasoning is more open-ended and has a high likelihood of producing a true conclusion if premises are true, deductive reasoning is more likely to promise truth. There are two types of deductive reasoning: syllogism and conditional reasoning. Deductive reasoning can be used:

  • in the workplace
  • in scientific research
  • in the classroom
  • in day-to-day life
  • in major decision making situations
  • in disagreements
  • … and many other settings!

Deductive reasoning helps individuals track their decision making process and make valid arguments grounded in factual premises. It makes daily life easier while also supporting policymakers, lobbyists, and other people in power make significant decisions.

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