Thinking Skills · 5 min

What must be true

The hardest word in a Thinking Skills paper is must. Four options can all sound sensible and only one is forced by what you were told. This is how to tell them apart.

The idea

A question asking what must be true is not asking what is likely, sensible or probably what the writer meant. It asks: is there any way the facts could all be true and this statement still be false? If you can imagine even one, it is out — however reasonable it sounds.

So the test is not “does this sound right?” It is try to break it. An option survives only if you cannot.

See it: three sentences, one arrangement

Add the premises one at a time and watch where each one forces the circles to go.

You try: sort each statement

Same three premises. For each statement, decide which bucket it belongs in — the exam only ever rewards the first one.

All teachers are readers.

Some readers are not painters.

Some teachers are writers.

All writers are readers.

Some writers are painters.

The three traps

Turning a sentence around.“All teachers are readers” does not give you “all readers are teachers”. One direction only, always.
Reading “some” as “some but not all”.In these questions “some” means at least one — it leaves “all” open.
Skipping the option that repeats a premise. It looks too easy to be the answer, so students walk past the one statement that literally cannot be false.

1. All dancers are fit. Some fit people run marathons. Which MUST be true?

2. No reptiles have fur. All geckos are reptiles. Which MUST be true?

Ready to try it against real exam questions?

Practise formal & conditional logic

162 questions in this topic, with a worked explanation on every one.