2. The Logic Condition

The Logic Condition is another necessary condition for an argument to be "good".

In the tutorial that introduced the notion of a "good argument" we defined the Logic Condition in very general terms: an argument satisfies the Logic Condition if the conclusion "follows from" the premises.

The main point I wanted to make in that discussion was to distinguish arguments that are bad because of false premises, from arguments that are bad because of bad logic. These are two distinct ways in which an argument can fail to provide good reasons to believe the conclusion.

But we need to say a lot more about the Logic Condition, and what it means to say that an argument has good logic.

In fact, this lecture and the next two lectures are devoted to it. The concepts that we'll be looking at are absolutely central to logic.

The Hypothetical Character of the Logic Condition

I want to highlight the hypothetical character of the Logic Condition, and how it differs from the Truth Condition.

We say that an argument satisfies the Logic Condition if the conclusion "follows from" the premises, or equivalently, if the premises "support" the conclusion.

The following two arguments give examples of good logic and bad logic, respectively.

1. All tigers are mammals.
2. Tony is a tiger.
Therefore, Tony is a mammal.

In this first argument the conclusion clearly follows from the premises. If all tigers are mammals and if Tony is a tiger then it follows that Tony is a mammal.

1. All tigers are mammals.
2. Tony is a mammal.
Therefore, Tony is a tiger.

In this second argument the conclusion doesn't follow. If all tigers are mammals and if Tony is a tiger we can't infer that Tony is a tiger. Those premises may be true but they don't support the conclusion.

I want to draw attention to the way in which we make these kinds of judgments. In judging the logic of the argument we ask ourselves this hypothetical question:

"IF all the premises WERE true, WOULD they give us good reason to believe the conclusion?"

If the answer is "yes" then the logic is good, and the argument satisfies the Logic Condition.
If the answer is "no" then the logic is bad, and the argument doesn't satisfy the Logic Condition.

This gives us a more helpful way of phrasing the Logic Condition. An argument satisfies the Logic Condition if it satisfies the following hypothetical condition:

If the premises are all true, then we have good reason to believe the conclusion.

The key part of this definition is the hypothetical "IF".

When we're evaluating the logic of an argument, we're not interested in whether the premises are actually true or false. The premises might all be false, but that's irrelevant to whether the logic is good or bad. What matters to the logic is only this hypothetical "if". IF all the premises WERE true, WOULD the conclusion follow?

So, when evaluating the logic of an argument we just ASSUME the premises are all true, and we ask ourselves what follows from these premises?

This is fundamentally different from the Truth Condition, where what we're interested in is the actual truth or falsity of the premises themselves (or as we talked earlier, the "plausibility" or "implausibility" of premises).

The Truth Condition and the Logic Condition are focusing on very different properties of arguments.

In fact you can have arguments with all false premises that satisfy the Logic Condition. Here's an example:

1. If the moon is made of green cheese, then steel melts at room temperature.
2. The moon is made of green cheese.
Therefore, steel melts at room temperature.

This argument satisfies the Logic Condition, even though both premises are clearly false. Why? Because IF the first premise WAS true, and IF the second premise WAS true, then the conclusion WOULD follow.

In fact, this argument is an instance of a well known argument FORM that always satisfies the Logic Condition:

1. If A then B
2. A
Therefore, B

If A is true then B is true; A is true, therefore B is true. ANY argument that instantiates this argument form is going to satisfy the Logic Condition.

Here's another example:

1. All actors are billionaires.
2. All billionaires are women.
Therefore, all actors are women.

Each claim in this argument is false, so it's a bad argument, but the logic is airtight. This argument fails the Truth Condition but satisfies the Logic Condition.

And like the previous example it's an instance of an argument FORM that always satisfies the Logic Condition:

1. All A are B.
2. All B are C.
Therefore, all A are C.

Alternately, you can have arguments that have all true premises but FAIL the Logic Condition, like this one:

1. If I live in Iowa then I live in the United States.
2. I live in the United States.
Therefore, I live in Iowa.

The premises are true (at the time of writing this), but the conclusion doesn't follow, because even if they're all true it doesn't follow that I have to live in Iowa, or even that it's likely that I live in Iowa. I might live in New York or Florida or any of the other fifty states.

So, to summarize:

We can rephrase the Logic Condition in a more helpful way by emphasizing the hypothetical character of the property that we're interested in, which is the LOGICAL RELATIONSHIP between premises and conclusion: If the premises are all true, then we have good reason to accept the conclusion.
The actual truth or falsity of the premises is irrelevant to the logic of the argument.
Argument analysis is a two-stage process. When we evaluate the logic of the argument we're not concerned about the actual truth or falsity of the premises. All we're concerned with is whether the conclusion follows from those premises.
Once we've evaluated the logic, then we can ask whether the premises are actually true or plausible.
If you confuse these steps, and make the mistake of judging the logic of an argument in terms of the truth or falsity of the premises, then you won't be able to properly evaluate an argument.

This distinction, between TRUTH and LOGIC, is arguably the most important distinction for critical argument analysis.