In IRAC you applied a rule to facts. Underneath, that is an old form of argument from logic. If you have met modus ponens on the Logic Ladder, you already know its shape: if P then Q; P; therefore Q.
Key idea
Applying a rule is a deductive argument, often called a legal syllogism: the rule is the major premise, the facts are the minor premise, and the result follows. Most legal disputes are not about the logic but about the premises: which rule applies, what it means, and what the facts are.
| Part | Logic | Law |
|---|---|---|
| Major premise | If P, then Q. | Anyone who takes another's property without consent, intending to keep it, commits theft. |
| Minor premise | P. | Kim took Lee's bicycle without consent, intending to keep it. |
| Conclusion | Therefore Q. | Kim committed theft. |
Why it works
Why frame law this way? Because it makes the reasoning checkable. If the premises are true and the form is valid, the conclusion must follow, so anyone who rejects the conclusion has to say which premise is wrong. That points the argument where it belongs. A valid argument can still be unsound if a premise is false. So lawyers rarely attack the form. They attack the major premise (that is not the rule here, or the rule means something else) or the minor premise (that is not what happened, or that fact does not fit the rule's words).
Worked example: Where is the fight?
Rule: "No vehicles in the park." Facts: Ada rode a bicycle through the park. Conclusion: Ada violated the rule. The syllogism looks valid. Where will the argument be?
- The form is fine: if a vehicle is in the park, the rule is broken; a vehicle was in the park.
- But the minor premise hides a question: is a bicycle a "vehicle" under this rule?
- That is a question about the rule's meaning, not about logic. (Legal philosophers have used this "no vehicles in the park" example since H. L. A. Hart used it in 1958.)
Answer: Over the meaning of "vehicle," a premise, not the logic.
Worked example: Affirming the consequent
A lawyer argues: "Anyone who commits burglary enters a building. My client's neighbor entered the building. So the neighbor committed burglary." What is wrong?
- The rule says burglary → entry, not entry → burglary.
- Inferring the "if" part from the "then" part is the fallacy of affirming the consequent.
- Many people enter buildings lawfully.
Answer: The argument is invalid: it affirms the consequent.
Common mistake
Thinking a valid legal syllogism settles a case. The form may be valid, but each premise can be contested: the rule's existence and meaning, and the facts and how they are described. Check: when you lay out a syllogism, ask of each premise, "could the other side deny this?" That is where the case will be argued.
This is an educational course, not legal advice.