How do you know if a claim is true?

There are many ways arguments can go wrong, but only a few ways to make them logical.  Logical arguments provide convincing evidence for claims.  What kind of evidence counts depends on what kind of claim has been made.

Opinions are never false, because the evidence is in the mind of whoever is giving the opinion.  For example:

I don't like to eat green vegetables.
Is that true or false?  To find out, you'd have to be inside the body of the person who said it.  Since that's impossible, there is no reason to question it.  Of course, opinions don't count for much when someone is trying to persuade you.  You can always answer, "I have a different opinion."

To decide if the evidence is convincing, you first have to know what sort of claim has been made.  Claims come in at least four types.

An empirical claim makes a statement about the world.  For example:

The moon is made of green cheese.
We need scientific knowledge about the world to test an empirical claim.  Scientific knowledge is public information gained by careful observations and experiments.  We have lots of evidence that the moon is made of rock, including the close-up observations of astronauts, so we know that the green-cheese claim is false.

An analytical claim makes a statement about the meaning of words or other symbols.  For example:

The Constitution gives us freedom of speech.
We need knowledge about words and symbols to test an analytical claim.  We might consult a document and use a dictionary or other reference to find out how people have agreed to interpret a word.  In this case, the claim is true because free speech is guaranteed in the First Amendment to the Constitution.

An evaluative claim makes a statement about what is good or bad, right or wrong.  For example:

People should read books instead of watching so much TV.
To test an evaluative claim, we appeal to standards of value.  In this case, the standard might be the value of literacy.  Evaluative claims often carry assumptions about empirical claims not directly stated.  Here, we are assuming that reading books makes us more literate than watching TV, which according to scientific studies of vocabulary growth, is also true.  Answering evaluative claims requires us to decide which value standard is higher.  In this case, we might argue that literacy is a higher standard than relaxation or pleasure.

A metaphysical claim makes a statement about our very existence.  For example:

All men are created equal.
To test a metaphysical claim, we appeal to revelation, that is, to statements of faith.  Reconciling conflicting metaphysical claims usually requires that we appeal to a common revelation.  For example, if we understand that the introduction to the Declaration of Independence expresses an essential truth about our existence on earth, then it is true that all men are created equal.  But if someone disputes the authority of the Declaration, we might not be able to resolve the question of whether all people are equal or not.  We may have to agree to disagree, because our opponent does not share our faith.

When we've stripped down an argument to the bare essentials--when it's stated in neutral, unemotional language, it's free of opinions, and we are willing to grant the authority and impartiality of the speaker--then our final questions are:

This is how we get at the truth, the whole truth, and nothing but the truth.

Click here to see some examples and try your hand at evaluating the truth of claims.

Click here to find out about some popular fallacies, or ways arguments go wrong.

Click here to return to the quotes from Nothing But the Truth.