Building an argument
Take any argument apart into premises and a conclusion, find the premise nobody said out loud, and tell whether the conclusion really follows.
Argument
In philosophy an argument isn't a quarrel. It's a set of statements, the premises, offered as reasons to accept another statement, the conclusion. Words like 'so', 'therefore' and 'that's why' usually point to the conclusion; 'because' and 'since' usually point to a premise.
'The bus is late every morning, so I'll leave ten minutes earlier.' Premise: the bus is late every morning. Conclusion: I'll leave ten minutes earlier.
Neda, in the family group:
"Dad shouldn't buy a brand-new car. A new car loses
a big chunk of its value in the first few years,
and he only drives to work and back. A good used
one would do the same job for much less."P1 A new car loses a big chunk of its value early.
P2 Dad only drives short, simple trips.
P3 A used car would do those trips for much less money.
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C Dad shouldn't buy a brand-new car.
Now each line can be checked on its own.Writing it as numbered premises above a line and the conclusion below is the standard way philosophers lay an argument out.
Check yourself
Kian texts: 'Since the exam covers chapters 1 to 6 and I've only done three chapters, I'm skipping the party on Thursday. Also the party is far away.' What is his conclusion?
- The exam covers chapters 1 to 6
- He has only studied three of the six chapters so far
- The party is far away
- He is skipping the party on Thursday
Show the answer
He is skipping the party on Thursday
Yes. Everything else is a reason for that decision. 'Since' flags the first premise and 'also' adds one more.
Two separate questions to ask
Valid
Asks: if the premises were true, would the conclusion have to be true? It's about the link, not the facts. An argument can be valid with a false premise.
Sound
Asks: is it valid and are all the premises actually true? Only a sound argument guarantees its conclusion. This is what you want.
SEE THE DIFFERENCE
Good links, bad links
Valid but unsound: 'All birds can fly. Penguins are birds. So penguins can fly.' The link is perfect; the first premise is false.
True premises, invalid link: 'If it rained, the street is wet. The street is wet. So it rained.' Both premises can be true while the conclusion is false: a water truck could have passed. This pattern is so common it has a name, affirming the consequent.
'If he's angry, he doesn't reply. He hasn't replied. So he's angry.' Same broken shape. Maybe his phone is dead.
Check yourself
Sort each argument
- Every month has at least 28 days. February is a month. So February has at least 28 days.
- All fish breathe air. Sharks are fish. So sharks breathe air.
- If I have flu, I have a fever. I have a fever. So I have flu.
- Tehran is the capital of Iran. Tehran is in Iran. So Tehran is the largest city in Asia.
- Everyone who studies hard gets top marks. Leila studies hard. So Leila will get top marks.
- All squares have four sides. This shape is a square. So it has four sides.
Show the answer
Sound: Every month has at least 28 days. February is a month. So February has at least 28 days., All squares have four sides. This shape is a square. So it has four sides.
Valid, but a premise is false: All fish breathe air. Sharks are fish. So sharks breathe air., Everyone who studies hard gets top marks. Leila studies hard. So Leila will get top marks.
Conclusion doesn't follow: If I have flu, I have a fever. I have a fever. So I have flu., Tehran is the capital of Iran. Tehran is in Iran. So Tehran is the largest city in Asia.
THE MISSING PIECE
The hidden premise
Everyday arguments almost always skip a step. The skipped step is the hidden premise (logicians call an argument with one an enthymeme). Finding it is often the most useful thing you can do, because the hidden premise is usually where the real disagreement lives.
'Hamed got top marks in biology, so he should study medicine.' Hidden premise: anyone who is good at biology should become a doctor. Said out loud, most people wouldn't accept it.
Checking any argument
- Find the conclusion
What am I being asked to believe or do?
- List the stated premises
Numbered, one idea per line, in plain words.
- Add the hidden premise
What would have to be true for the conclusion to follow? Write it down.
- Test the link, then the facts
First: does it follow? Then: is each premise, especially the hidden one, actually true?
Check yourself
Farzaneh's aunt says: 'That café is always full, so the food must be excellent.' Which hidden premise does her argument need?
- A café that is always full serves excellent food
- Farzaneh should try the café this weekend
- The café has been open for many years
- Full cafés are usually loud and it's hard to find a table
Show the answer
A café that is always full serves excellent food
Yes. Without that premise, 'full' says nothing about 'excellent'. Once it's said aloud, you can see how shaky it is: a café can be full because it's cheap, central or near a university.
PLAY FAIR
The principle of charity
When you rebuild someone's argument, fill the gaps with the most reasonable version they could mean, not the silliest. Philosophers call this the principle of charity. It's fairer, and it's smarter: if you can answer their best version, you've really answered them. If you can't, you've learned something.
Farzaneh's aunt might really mean 'a place locals keep coming back to is probably decent'. That's a much better argument than 'crowds are never wrong', and worth taking seriously.
Check yourself
Omid shows that his cousin's argument for moving abroad has an invalid step. That proves moving abroad is a bad idea for the cousin.
Show the answer
False
False. A bad argument for a conclusion doesn't make the conclusion false. It only means this argument doesn't support it. There might be good reasons for the cousin to move that nobody has stated yet. Treating a weak argument as proof of the opposite is a common trap, and it comes up again in the next lesson.
Lesson recap
- An argument is premises offered as reasons for a conclusion.
- Valid means the conclusion follows; sound means valid and the premises are true.
- Most everyday arguments have a hidden premise, and that's usually where the disagreement is.
- Rebuild other people's arguments charitably, in their strongest form.
- A bad argument for something doesn't prove it false.