Core Idea
Definition
Deductive reasoning infers conclusions that are logically entailed by a set of premises, guaranteeing truth preservation from premises to conclusion.
In Plain English
Deduction starts with general rules or premises and applies them to reach conclusions that cannot be false if the premises are correct.
Framework Structure
Components
Flow
Premises → Apply logical rules → Necessary conclusion
How to Apply
- 1.Clearly state all premises explicitly
- 2.Check that premises are factual or assumed by agreement
- 3.Apply valid logical rules (e.g., modus ponens, syllogism)
- 4.Derive the conclusion
- 5.Verify that the conclusion follows necessarily from the premises
When to Use
- •Formal argument evaluation
- •Mathematical or logical proofs
- •Policy or legal reasoning
- •Checking internal consistency of arguments
- •Any context requiring certainty rather than probability
When NOT to Use
- •When premises are uncertain or incomplete
- •When reasoning from limited observations
- •When dealing with probabilistic or noisy data
- •When causal mechanisms are unknown
Example
Problem
Is Socrates mortal?
Application
- 1.Premise 1: All humans are mortal
- 2.Premise 2: Socrates is a human
- 3.Apply syllogistic logic
Conclusion
Socrates is mortal
Takeaway
If the premises are true, the conclusion cannot be false.
Common Mistakes
- •Assuming true conclusions guarantee true premises
- •Using invalid logical forms
- •Hiding implicit premises
- •Confusing validity with soundness
- •Applying deduction to empirical uncertainty
How to Practice
argument skeleton builder
Practice explicitly separating premises from conclusions.
fallacy flash drills
Identify invalid deductive forms and hidden assumptions.
reverse argument exercise
Test whether conclusions still follow when premises are altered.
Related Cognitive Biases
belief bias
People judge arguments by whether they like the conclusion rather than whether the logic is valid.
confirmation bias
People accept deductions that support their beliefs and scrutinize opposing ones more harshly.
illusion of explanatory depth
People believe they understand logical arguments better than they actually do.
Related Frameworks
Related Skills
Variants & Extensions
Typical Failure Modes
- •Invalid logical form
- •False premises
- •Implicit assumptions
Further Reading
- An Introduction to Logic by Irving M. Copi
- Logic: A Very Short Introduction by Graham Priest
- Being Logical by D.Q. McInerny