Completing an Argument: Finding the Missing Premise or Conclusion

An argument, in the logical sense, is a set of premises (statements taken as true) that lead to a conclusion. In the IMAT's classic "argument completion" exercise, you are given a passage missing one element — often an implicit premise, sometimes the conclusion itself — and asked to choose, among 5 options, the one that makes the argument logically complete.

The method is to treat the argument as a chain: each premise is a link, and the conclusion must follow necessarily from the whole chain. If a link is missing, the chain is broken — your task is to find the missing link, not to add a true but unnecessary piece of information.

Simple example: "All marine mammals must surface to breathe. Dolphins are marine mammals. Therefore dolphins must surface to breathe." Here, the two premises are enough to guarantee the conclusion — the argument is complete. If the first premise were removed, the link between "marine mammal" and "needing to surface to breathe" would be missing: that is exactly the kind of missing link the IMAT asks you to spot.

Illustration: the structure of an argument to complete

Method and a fully solved example

Three traps come up constantly in this type of exercise: (1) confusing a necessary condition with a sufficient one — "B is necessary for A" ($A \rightarrow B$) does not mean "B is sufficient for A" ($B \rightarrow A$); (2) affirming the consequent — from "if $P$ then $Q$" and "$Q$ is true", you CANNOT conclude $P$ is true ($Q$ could have other causes); (3) smuggling in an unstated premise that sounds "reasonable" but isn't given in the text.

Example: "All certified employees wear a blue badge. No one without a blue badge is allowed in the laboratory. Marc does not have a blue badge." What can be concluded with certainty? Step 1: translate into implications — certified → blue badge; no blue badge → not allowed in the lab. Step 2: apply modus ponens to the second implication with the given fact (Marc has no blue badge) → Marc is not allowed in the laboratory. That is the only valid deduction. An answer like "Marc is not certified" is a classic trap look-alike: it's actually a valid modus tollens on the FIRST implication (no blue badge → not certified), so it is also derivable — but an answer like "Marc works elsewhere" would smuggle in information absent from the text, and must be rejected.

• Premise: a statement taken as true, serving as a starting point for the reasoning.
• Conclusion: what the argument seeks to establish from the premises.
• Valid argument: an argument where the conclusion follows necessarily from the premises (regardless of whether the premises are actually true).
• Implicit (or "missing") premise: a statement not stated but necessary for the argument to be valid.

Think of the argument as a bridge between two shores: the premises are the starting shore, the conclusion the arrival shore. If a pillar is missing, you cannot cross — even if everything written is otherwise true. Your only question should be: "what, added to the given premises, makes the conclusion inevitable?" — not "what is true and relevant to the topic?".

• Choosing a true but unnecessary statement: an option can be factually correct without filling the argument's logical gap. The IMAT often baits with this kind of "realistic" distractor.
• Adding too much information: the right answer fills exactly the gap, no more, no less. An option that goes beyond what is necessary is usually not the best answer.
• Confusing "strengthening" and "completing": strengthening an argument (making it more likely) is not the same as completing it (making it logically necessary).