Thinking Skills · Module 11
Spotting Assumptions & Flaws
Find the hidden assumption and the weak link in an argument — real critical thinking, with four tiers of practice.
Page 1 · Introduction
Spotting assumptions & flaws
An argument tries to convince you of something. But many arguments quietly rely on a hidden assumption — something taken for granted that might not be true. Learning to spot these is real critical thinking.
This module trains you to find the hidden assumption, and to spot when reasoning is weak or jumps to a conclusion.
The big ideaAn assumption is the missing link an argument needs to work. If that link is shaky, the whole argument is shaky.
💡This is a "slow down and think" topic. We build from simple assumptions up to spotting flaws in trickier arguments.
On the real testWhere this appearsIdentifying assumptions and flaws is a named part of the Thinking Skills section (25% of your mark, from 2025) — the "critical thinking" style question.
Page 2 · The parts of an argument
Reasons and conclusions
Most arguments have two parts:
The reason (evidence)
The fact given as support. "It's raining."
The conclusion (the point)
What they want you to accept. "So the match will be cancelled."
The jump from reason to conclusion often hides an assumption — here, "rain always cancels the match", which may not be true.
✅First find the conclusion (the main point), then ask: "what has to be true for this to follow from the reason?"
Page 3 · The hidden link
Finding the assumption
The assumption is the unstated fact the argument needs. Test it: if the assumption were false, would the argument fall apart?
"Sam trains every day, so he will win the race." What is assumed?
The argument needs: training every day is enough to win (and others don't train as much / aren't faster). If that's false, he might not win.
That daily training guarantees (or is enough for) winning
💡Handy test: "Does the conclusion REALLY follow, or does it need an extra unstated fact?" That extra fact is the assumption.
Page 4 · Weak reasoning
Common flaws to spot
Jumping to a conclusion
One example doesn't prove a general rule. "My dog is friendly, so all dogs are friendly."
Confusing "after" with "because"
"I wore red socks and we won, so my socks caused the win." Two things happening together doesn't mean one caused the other.
Ignoring other explanations
"The plant grew, so the new fertiliser worked." Maybe it was the sunshine, not the fertiliser.
Page 5 · Strengthen or weaken
What would change the argument?
Some questions ask what would weaken (or strengthen) an argument. A weakener attacks the hidden assumption.
"The cafe must be good — it's always busy." What would most weaken this?
It relies on "busy = good". A weakener: "it's the only cafe for miles, so people have no choice." That attacks the assumption.
Showing people go there for another reason (no choice)
✅To weaken an argument, find its assumption and show it might be false. To strengthen, show the assumption holds.
Page 6 · A key habit
Judge the argument, not the topic
Your job is to judge whether the reasoning works — not whether you agree with the conclusion. A true conclusion can still have a flawed argument.
Careful"Exercise is healthy" is true — but "my uncle exercised and lived to 90, so exercise makes you live to 90" is still a flawed argument (one example proves nothing). Judge the logic, not the claim.
💡Ask "does the reason actually prove the conclusion?" — not "do I think the conclusion is true?" They're different questions.
Page 7 · Practice
Level 1 · Easy
Warm up
Find the assumption or the conclusion. Tap to check.
1. "It's cloudy, so it will rain." What is the conclusion?
A It's cloudy
B It will rain
C Clouds are grey
D It is cold
The conclusion is the point being argued: "it will rain". "It's cloudy" is the reason.
2. "Mia is tall, so she must be good at basketball." What is assumed?
A Mia is short
B Basketball is fun
C Tall people are good at basketball
D Mia likes sport
The argument only works if "tall means good at basketball" — that's the hidden assumption (and it isn't always true).
3. "Every swan I've seen is white, so all swans are white." What is the flaw?
A A few examples don't prove a general rule
B Swans can't fly
C White is not a colour
D There are no swans
Seeing some white swans doesn't prove ALL are white (black swans exist!). Jumping from a few examples to "all" is the flaw.
🎉You're already spotting hidden links and weak jumps. Let's go deeper.
Page 8 · Practice
Level 2 · Medium
Spotting the flaw
1. "We won the game after I wore my lucky hat, so the hat made us win." What is wrong with this?
A The hat was too big
B Two things happening together doesn't mean one caused the other
C Hats can't be lucky or unlucky — so we won
D The game wasn't important
Wearing the hat and winning happened together, but that doesn't prove the hat caused the win. Confusing "after" with "because" is the flaw.
2. "This medicine is popular, so it must work well." What is assumed?
A Medicine is expensive
B Popular things are cheap
C Popular things must be effective
D People are unwell
The argument assumes "popular = works well" — but popular things aren't always effective. That's the hidden assumption.
3. "The garden grew well this year. I used a new fertiliser, so the fertiliser caused it." What has been ignored?
A Other causes, like better weather
B The garden is small
C Fertiliser is expensive
D Plants need water
The good growth might be due to sunshine, rain or something else — the argument ignores other possible causes.
💪"After isn't because" and "ignoring other causes" are two of the most common flaws. Spot them and you're thinking critically.
Page 9 · Practice
Level 3 · Hard
Strengthen & weaken
1. "The library must be the quietest place in town — it's always silent when I visit." What would most WEAKEN this?
A The library has many books
B The person only ever visits at 6am, before it opens to others
C The library is large
D Libraries are usually quiet
If they only visit at an unusually quiet time, their evidence doesn't show it's always the quietest place. That attacks the assumption behind the argument.
2. "Students who eat breakfast score higher, so eating breakfast makes you smarter." What would most WEAKEN this?
A Breakfast is tasty
B Some students skip breakfast
C Students who eat breakfast also tend to sleep more, which could be the real cause
D Breakfast is the first meal
If breakfast-eaters also sleep more, the higher scores might come from sleep, not breakfast. Offering another cause weakens the argument.
3. "Our new coach arrived and the team started winning, so the coach is the reason." What would most STRENGTHEN this?
A Nothing else about the team changed when the coach arrived
B The coach is friendly
C The team has new uniforms
D The coach was once a player
If nothing else changed, it's more likely the coach made the difference — that supports the assumption. (Option C would actually weaken it by adding another cause.)
💡To weaken: offer another cause or attack the assumption. To strengthen: rule other causes out. This is exactly how the test's critical-thinking questions work.
Page 10 · Challenge
Level 4 · Challenge
Think smart
Trickier arguments. Read carefully and find the exact assumption or flaw.
1. "No one has ever proved ghosts don't exist, so ghosts must be real." What is the flaw?
A Ghosts are scary
B The sentence is too long
C Not being able to disprove something doesn't make it true
D Some people believe in ghosts
"You can't prove it's false, so it's true" is a classic flaw. A lack of disproof is not proof.
2. "Most successful people wake up early. So if I wake up early, I will become successful." What is assumed?
A Early mornings are cold
B Waking early is what causes the success (not just linked to it)
C Successful people are tired
D Everyone can wake early
It assumes waking early causes success. But maybe successful people wake early for other reasons — the link doesn't prove the cause.
3. "This test is easy. My friend passed it, and she's not even very good at maths." What must be assumed for this to make sense?
A If someone not good at maths can pass, the test must be easy
B The friend cheated
C Maths is unimportant
D The test was long
The argument only works if "someone weak at maths passing" proves the test is easy — that's the assumption. (Maybe she studied hard, or is better than thought.)
🌟Spotting these subtle flaws — "can't disprove = true", "linked = caused" — is advanced critical thinking. It'll serve you far beyond any test.
Complete
Clear thinking!
You've completed the Spotting Assumptions & Flaws module.
You can now find the conclusion, uncover the hidden assumption, spot common flaws (jumping to conclusions, "after ≠ because", ignoring other causes), and weaken or strengthen an argument. That's genuine critical thinking.