Two parts, and two separate checks.
The idea
"Let's eat early, since the film starts soon." Nobody is shouting, but that's an argument. In everyday talk, "argument" often means a quarrel. In logic, an argument is a claim plus the reasons offered for it. Both parts can be true or false, so both can be checked. Small words help you find them. "Because" and "since" often mark a reason; "so" and "therefore" often mark the claim. They're clues, not rules, because those words do other jobs too: "since Monday" is about time, not a reason. In the film example, the claim ("Let's eat early") comes first, and the reason follows "since".
An argument can also lean on a reason nobody says out loud. "He's a pilot, so he won't mind the bumpy flight" rests on an unsaid one: pilots don't mind bumpy flights. Said out loud, that hidden reason can show where the real disagreement is. You might agree with every spoken reason and still doubt the unsaid one. Filling it in the way the speaker would is the fairest reading.
Two separate questions sit inside an argument that aims to prove its claim, not just make it likely. First, if the reasons were true, would the claim have to follow? If so, the argument is called valid. Second, are the reasons true? A valid argument with true reasons is called sound. So "valid" doesn't mean the ending is true. "All toasters are made of gold; everything made of gold is a time-travel device; so all toasters are time-travel devices." The steps connect perfectly, but the reasons are false, and so is the ending. It works the other way too: an argument whose steps don't connect can still end on a true claim. Spotting a flaw shows the argument doesn't prove its claim, not that the claim is false.
There's a quick test for whether the steps connect: keep the shape, swap the words. "Good leaders are confident; he's confident; so he's a good leader" can sound fair. Swap the words, keeping the shape (good leaders → basketballs, confident → round, he → the Earth): "Basketballs are round; the Earth is round; so the Earth is a basketball". Same shape, true reasons, a false ending, so the shape doesn't work. The test helps because a believable ending pulls hard. In studies of short logic puzzles, people accepted arguments more often when they already believed the ending. That held even when the steps didn't work. This pull is called belief bias.
Want the research, names and dates? They are in Go deeper, at the end.
Good recipe, sour milk.
Picture it
Think of a recipe and the food that goes into it. A good recipe with sour milk still makes a bad cake. A muddled recipe can still turn out a fine cake, by luck. The recipe is the argument's shape, and the ingredients are its reasons. An argument needs both: steps that connect, and reasons that are true.
For a child: Your little sister says, "I should get the last cookie, because I'm wearing my red sweater." You ask why the sweater matters. "Because red sweaters get cookies!" Now the hidden rule is out, and everyone can see the problem.
From what you are told to what you check.
How it applies
Looked at piece by piece, an argument gets easier to judge. The claim tells you what the reasons have to carry. The reasons, said and unsaid, show what it all rests on. Take "The shop will be shut, because it's a holiday." The claim is that it will be shut. The unsaid reason is that this shop shuts on holidays. Two questions then do the rest: whether the reasons would lead to the claim, and whether they're true. Here the claim would follow. The argument works only if this shop really does shut on holidays. Swapping in other words while keeping the shape helps with the first question. When the reasons are true and the claim has to follow from them, the claim is true as well. Some arguments only aim to make a claim likely, and the next lesson looks at those.
At work, online and at home.
Where you’ll see it
- At work: "The new logo went live and sales fell, so the logo hurt sales." Both facts can be true, and the claim still not follow.
- Online: a post you already agree with gets a quick share, its reasons unread. The ending felt right, so the steps went unchecked.
- At home: "You forgot to take out the trash again, so you don't care." It leans on an unsaid rule: forgetting means not caring.
Signs it’s happening
- You can say what you're meant to believe, but not the reasons for it.
- The argument only works if something nobody said is true.
- A string of true facts ends on a claim none of them quite reach.
Same neat post, two different readings.
Before and after
A post makes a neat case for something you already believe.
"Exactly right." It gets a nod, mostly because you already agreed with where it ended.
"Would these reasons convince me if the ending were one I disliked?" A neat case for something you believe still needs its steps checked.
A medieval book that shows its reasons.
A story from history: A round Earth, with its reasons
- In the 13th century, a scholar known as Sacrobosco wrote an astronomy book, De sphaera. It had a section headed "The Earth a Sphere".
- It states its claim plainly: "That the earth, too, is round is shown thus."
- Then it gives reasons. One is that the same eclipse of the Moon is seen at different hours in different places.
- It even answers the obvious doubt. The Earth "seems flat to human sight because it is so extensive."
- Separately, Saint Augustine, one of the early Church Fathers, made a different point in The City of God. Even if the world is round, he wrote, it does not follow that anyone lives on its far side. He himself rejected the idea that anyone did.
Sacrobosco set out a claim, its reasons and an answer to the obvious doubt. Augustine's point was about what follows. Even a true reason proves only the claims that really follow from it. Whether anyone lived on the far side was a separate question, needing reasons of its own.
Quick check3 questions · nothing is saved
1. An argument's reasons would lead to its claim, but one reason is false. What follows?
2. In "We should take umbrellas, because the forecast says rain", which part is the claim?
3. When you keep an argument's shape and swap the words, what shows the shape doesn't work?
One question worth keeping.
In one line
Two checks, one argument. An argument holds up when its reasons are true and really lead to its claim.
Which argument have you agreed with lately, mostly because you already liked where it ended?
Go deeperThe research, names and dates
Where this comes from
The Internet Encyclopedia of Philosophy points out that "argument" can name a fight, or be used in a technical sense. In that technical sense, statements called premises are offered as grounds for another statement, the conclusion. Words such as "because" and "since" typically flag premises; "therefore" and "so" typically flag the conclusion. The encyclopedia adds that these words don't always play those roles.
When at least one part of an argument goes unstated, logicians call it an enthymeme.
In the encyclopedia's terms, a deductive argument is valid when its form rules out true premises with a false conclusion. It is sound when it is valid and every premise is true. Its example of valid but unsound: toasters are all made of gold; whatever is gold is a time-travel device; so toasters are time-travel devices. Invalid arguments, and valid but unsound ones, can still reach true conclusions. Finding a flaw in an argument is no reason, on its own, to reject its conclusion.
Most logicians treat validity as a matter of form alone: what is left once the topic words are removed. In modern logic, one example of a form with true premises and a false conclusion shows it is invalid. The ancient philosopher Aristotle, too, proved invalidity with counterexamples. A paper by D. Trippas and colleagues calls belief bias one of the most-studied biases in reasoning. A syllogism is a three-line argument of one very specific form. A syllogism with a believable conclusion is accepted more often than the same form with an unbelievable one. Several competing theories have been proposed to explain why.
Valid vs sound
| Compared | Valid | Sound |
|---|---|---|
| The question | If the reasons were true, would the claim follow? | That, and are the reasons true? |
| What it judges | The shape of the reasoning | The shape, plus the facts |
| Can the claim be false? | Yes, if a reason is false | No, when every reason is true |
From the Internet Encyclopedia of Philosophy, for arguments that aim to prove their claim. Most logicians hold that validity depends on the form alone.
More examples
Some everyday lines, and the reason nobody said.
- If someone says “It’s on sale, so it’s a good buy”, the unsaid reason is “anything on sale is good value”.
- If someone says “She’s quiet, so she must be shy”, the unsaid reason is “quiet people are shy”.
- If someone says “Everyone’s booked it, so it must be good”, the unsaid reason is “what’s popular is good”.
What if…?
- What if it's an explanation, not an argument?
- Explanations look similar but are not usually counted as arguments. "It was heated, and all metals expand when heated" helps you understand why a metal expanded. It doesn't offer grounds to believe that it did.
- What if the reasons only make the claim likely?
- Then valid and sound aren't the words to use. Such arguments are judged strong or weak, and a strong one with true reasons may be called cogent. The next lesson looks at this kind of reasoning.
- What if I fill in the unsaid reason badly?
- Then you may end up arguing with a weaker view they don't hold. An earlier lesson, "How Do You Argue Well?", calls that a straw man.
Words you may meet
- argument
- In logic, a set of reasons (premises) offered in support of a claim (the conclusion). Not a quarrel. Glossary →
- valid argument
- An argument whose form makes it impossible for the premises to be true and the conclusion false. Its premises may still be false. Glossary →
- belief bias
- The tendency to accept an argument when its conclusion is believable, and reject it when it is not, whatever its form. Glossary →