The core of any debate is the arguments being used. To win a debate, you must show that your arguments are good, and your opponents arguments are bad. It sounds simple, but most people struggle to distinguish good and bad arguments. More often than not, these terms are used subjectively, resulting in widespread disagreement about whether an argument is good or bad. In reality, these terms are completely objective, and it is possible to know and demonstrate for sure whether an argument is good or bad.

There are three criteria for an argument to be good (note: I am dealing with deductive arguments here).

1. It contains only true premises.
2. It does not contain any logical fallacies.
3. The conclusion follows necessarily from the premises (technically, if #3 is violated, it’s a non-sequitur fallacy, so its redundant with #2, but it’s such an important point that I included it as its own criterion).

Any argument which does not meet all three criteria is a bad argument. If an argument is good, then you MUST accept its conclusion. If an argument is bad, then you MUST reject the argument. This is an important distinction. If the argument is good, then the conclusion must be true, but if the argument is bad, the conclusion may or may not true, all that you can conclude is that the argument itself does not work. For example:

1.  All men are mortals
2. Socrates is a man
3. Therefore, Socrates is a mortal.

This is a good argument. All the premises are true, there are no logical fallacies, and the conclusion follows necessarily from the premises. Socrates MUST be a mortal, there are no other possibilities. This is not an opinion, it’s a logical certainty. The following argument is, however, bad:

1. All men are mortals
2. Socrates is a mortal
3. Therefore, Socrates is a man.

This argument doesn’t work, because the conclusion does not follow necessarily from the premises (in technical terms this argument commits a fallacy known as affirming the consequent). The fact that Socrates is a mortal does not automatically mean that he is a man. Notice, however, that even though the argument does not work, the conclusion is actually true, we just can’t use this argument to get to that conclusion. That isn’t always the case, however. Consider:

1.  All men are mortals
2. Trogdor (my pet gecko) is a mortal
3. Therefore, Trogdor is a man.

Now the problem with the argument is even more obvious because the conclusion is not true. This brings me to the restatement of a very important point. If someone demonstrates that one of your arguments is bad, you MUST reject the argument. From this example, it should be obvious that a bad argument tells you nothing about the conclusion, and continuing to use an argument that you know is bad is ridiculously foolish.

There is one and only exception to the rule that you reject a bad argument, not its conclusion. This occurs when the argument is absolutely essential to your opponents position. Under that condition, demonstrating that the argument is bad also demonstrates that the conclusion is wrong, but that is a fairly rare occurrence.

Note: what I have just described applies only to deductive arguments. These are the most common and most powerful types of arguments because they show what absolutely must be true. Other types of arguments, like inductive arguments, show what is probably true, not what must be true. So for an inductive argument to be good, it must contain only true premises, have no logical fallacies, and the conclusion must be the most likely outcome of the premises. So for inductive arguments, you accept the conclusion as the most likely answer, not as the definite answer. However, it is still logically invalid to reject that conclusion unless you can demonstrate that a premise is false, a fallacy has been committed, or another answer is more likely.

Other posts on the rules of logic:

• The Rules of Logic Part 1: Why Logic Always Works
• The Rules of Logic Part 3: Logical Fallacies
• The Rules of Logic Part 4: The Laws of Noncontradiction and Transitive Properties
• The Rules of Logic Part 5: Occam’s Razor and the Burden of Proof
• The Rules of Logic Part 6: Appealing to Authority vs. Deferring to Experts
• The Rules of Logic Part 7: Using Consistent Reasoning to Compare Apples and Oranges