1. The Truth Condition

The Truth Condition is a necessary condition for an argument to be good. We stated it as the condition that all the premises of an argument have to be true. In this lecture I want to talk about what this condition amounts to in real world contexts where arguments are used to persuade specific audiences to accept specific claims.

I'm going to try to show why we actually need to modify this definition somewhat to capture what's really important in argumentation. I'll offer a modification of the definition that I think does a better job of capturing this.

Here's our current definition of the Truth Condition:

All premises must be true.

I’m going to use a simple example to illustrate the problem with this definition.

Consider this claim:

"The earth rotates on its axis once every 24 hours".

We all agree that this claim is true. It's an accepted part of our modern scientific understanding of the world.

But, say, 500 years ago, this claim would have been regarded by almost everyone as obviously false. The common understanding was that the earth does not move. Most everyone believed that the planets and everything else in the universe revolved around the earth, which his fixed at the center of the universe.

And they had good reason to believe this. When we look outside we see the moon and the sun and the stars and planets all moving around us. It certainly doesn't seem like we're all moving at hundreds of miles an hour toward the east. Around the equator it would be closer to a thousand miles an hour, as fast as a rifle bullet. If the earth was really rotating that fast, why don't centrifugal forces make us fly off the surface of the earth? Or why don't we experience perpetual hurricane-force winds as the rotating earth drags us through the atmosphere at hundreds of miles per hour?

These are the sorts of arguments that medievals might have given, and did give, to support their contention that the earth in fact does not move. For their time, given what they knew about physics and astronomy, these seem like they would be compelling arguments.

So, for a medieval audience, any argument that employed this claim as a premise — the claim that the earth rotates once every 24 hours — or that argued for it as a conclusion, would have been judged as a bad argument, because for them the claim is clearly false.

Now, why does this situation pose a problem for our version of the Truth Condition? It poses a problem because if we read "true" as REALLY true, true IN ACTUALITY, and we think it's really true that the earth rotates, and it's really false that it's fixed at the center of the universe, then this version of the Truth Condition makes it so that no medieval person can have a good argument for their belief that the earth does not move.

And this just seems wrong. It seems like we want to say that yes, they were wrong about this, but at the time they had perfectly good reasons to think they were right.

And if they had good reasons, that means they had good arguments. But this version of the Truth Condition doesn't allow us to acknowledge that they had good arguments for their belief that the earth does not move.

So, we need to modify our phrasing of the Truth Condition so that it's sensitive to the background beliefs and assumptions of particular audiences.

This is a natural modification that does the trick:

We'll call a claim plausible (for a given audience — plausibility is always relative to a given audience even if we don't specifically say so) if that audience believes they have good reason to think it's true.

So to say that a claim is plausible for a given audience is just to say that the audience is willing to grant it as a premise, that they're not inclined to challenge it, since they think they have good reason to believe it's true.

What we're doing here is pointing out that in real-world argumentation, when someone is offering reasons for someone else to believe or accept something, an argument will only be persuasive if the target audience is willing to grant the premises being offered. Premises have to be plausible to them, not just plausible to the arguer, or some hypothetical audience.

So here are our conditions for an argument to be good, with the truth condition modified in the way that I've just suggested.

Condition one: The Truth Condition

All premises must be true (where “true” is read as “plausible”)

Condition two: The Logic Condition

The conclusion most follow from the premises.

Just to note, for the rest of these lectures I'll keep using the expression "Truth Condition", even though it's really a "plausibility” condition, just because the language of "truth" is so commonly used in logic and critical thinking texts when describing this feature of good arguments.

Just remember, when we talk about evaluating the premises of an argument to see if the argument satisfies the Truth Condition — when I give an argument and I ask, are all the premises true? — what I'm really asking is whether the intended audience of the argument would be willing to grant those premises. In other words, whether they would find those premises plausible.

A Possible Objection

Let me just wrap up with an objection to this modification that some of my students will usually offer at this point.

Some people might object that what I've done here is redefined the concept of truth into something purely relative and subjective, that I'm denying the existence of objective truth.

This isn't what I'm saying. All I'm saying is that the persuasive power of an argument isn't a function of the actual truth of its premises. It's a function of the subjective plausibility of its premises for a given audience. A premise may be genuinely, objectively true, but if no one believes it's true, then no one will accept it as a premise, and any argument that employs it is guaranteed to fail, in the sense that it won't be judged by anyone as offering good reasons to accept it.

This point doesn't imply anything about the actual truth or falsity of the claims. We can say this and still say that claims or beliefs can be objectively true or false. The point is just that the objective truth or falsity of the claims isn't the feature that plays a role in the actual success or failure of real world arguments. It's the subjective plausibility of premises that plays a role, and that's what this reading of the Truth Condition is intended to capture.