Deductive reasoning aims to establish a conclusion that must be true if its premises are true, whereas inductive reasoning uses evidence to establish a conclusion that is probably true. Deduction is assessed through validity and soundness; induction through strength and cogency.
Reasoning and Mechanism
A deductive argument is valid when it is impossible for all its premises to be true and its conclusion false. If it is valid and every premise is actually true, it is sound.
For example:
- All humans are mortal.
- Socrates is human.
- Therefore, Socrates is mortal.
The conclusion follows necessarily. Validity concerns logical structure, not whether the premises are actually true.
An inductive argument moves from observed evidence to a generalization or prediction:
- Every observed human has been mortal.
- Therefore, all humans are probably mortal.
This conclusion extends beyond the observations, so it is not logically guaranteed. Inductive arguments are strong when the premises make the conclusion highly probable and cogent when they are strong with true, well-supported premises.
| Feature | Deductive reasoning | Inductive reasoning |
|---|---|---|
| Aim | Necessity | Probability |
| Standard of assessment | Valid or invalid; sound or unsound | Strong or weak; cogent or uncogent |
| Typical movement | General principle to specific conclusion | Observations to generalization or prediction |
| New evidence | Does not alter a valid relationship | Can strengthen or weaken the conclusion |
| Key issue | Whether premises justify certainty | Whether limited evidence justifies broader beliefs |
A common misconception is that deduction always gives true conclusions. It does so only when the argument is valid and has true premises. Likewise, an inductive conclusion can be justified without being certain.
IB Exam Technique
When analysing an argument, identify its premises and conclusion before classifying it. Use validity for deductive structure and strength for inductive support; then assess whether the premises are true or justified. This distinction shows different degrees of justification.
