When I was a philosophy student in college, one of my professors articulated a principle of argument which I have remembered ever since: attack your opponent’s assumptions and his inferences. I have taught logic for some 15 years now, but when I find myself in an actual debate, this is the only thing I consciously bring with me.
This principle is based on a fundamental distinction in logic and this is reflected in the two branches of traditional logic. The first branch of traditional logic is formal logic. Formal logic has to do with the structure of an argument. In formal logic, truth is secondary, because the primary concern is whether the conclusion follows from the premises—whether the premises are true or false. In material logic, on the other hand, which is the second branch of traditional logic, it is the truth of the premises that is primary.
Let’s say I have the following argument:
Taking an innocent human life is wrongFormal logic asks whether the conclusion, “Abortion is wrong,” follows from the two premises, “Taking an innocent human life is wrong” and “Abortion is the taking of an innocent human life.” It does not ask whether the premises themselves are true or false. If the conclusion does follow, then we say the argument is valid. If it does not, it is invalid. The student studying formal logic would study the seven rules of validity. He would also memorize the nineteen valid argument forms as a sort of shortcut so he could see an argument’s validity at a glance.
Abortion is the taking of an innocent human life
Therefore, abortion is wrong
A good logic student would quickly see that the above argument is indeed valid: its conclusion does follow logically from the two premises.
But material logic would ask the further question: “Are the two premises really true?” Is it true to say that taking an innocent human life is wrong, and is it also true that abortion takes an innocent human life? Material logic, too, has its rules, which involve definition and division. If, in addition to being valid, the argument’s premises are true, then we say that the argument is sound.
When my philosophy professor said “attack the assumptions and the inferences,” he was assuming this distinction between an argument’s structure and its content. He was saying, in effect, “Ask if the premises in your opponent’s argument are true, and also whether or not the argument as a whole is valid.” By “assumptions” we mean the premises of the argument; by “inferences” we mean what would also follow from the premises if the argument were, in fact, valid.
Let’s try this method of attacking the assumptions and inferences on another argument we often hear today:
There is a right to same-sex marriageNow, I have never seen the argument stated exactly this way. Often it is stated in an invalid form (this is the case with most arguments you hear, unfortunately). But if I were an advocate of same sex marriage, this is the way I would state it. When Thomas Aquinas argued for the truths of Christianity, he always put his opponents’ arguments in the best light; he always gave them the benefit of the doubt, if it was possible. As a matter of ethics, we should always try to do this.
All rights should be protected by law
Therefore, the right to same-sex marriage should be protected by law
In the above case, then, how do we attack the assumptions and the inferences? Let’s attack the assumptions first.
The most common situation is that there is one true premise and one false premise, and often it is the first premise (called the “major” premise in logic) that is the problem. If we have stated the argument properly and put the major premise (the most general one) first, that is the most likely place our opponent may have gone wrong. If we apply that to this argument, then we want to look at the premise that states that same-sex marriage is a “right.”
Well, is it?
Notice that at this point we seem to have gone well outside the boundaries of logic. A premise in an argument can be gotten from anywhere. It could be a statement of science, or of history, or of economics. But there are still principles of material logic we can apply here. One of the aspects of material logic is definition, which has to do simply with what words mean. In this case, we can ask whether, given the meaning of the word, same-sex marriage could qualify as a “right” or not.
So the first question to ask is, “What is a right, and how do we know if something qualifies as one?” There are many ways of addressing this question. Just for fun, I am going to employ the most sophisticated and beautiful of all logical arguments: the dilemma. The dilemma is a way of putting your opponent in a box; it is a way of showing him that, no matter what in fact is the case, his assumption leads to an unacceptable conclusion. Again, there are numerous ways of attacking the truth of a statement—this is only one of them.
If I am making this argument, here’s how I do it: There are only two kinds of rights: those that originate in divine law and those that originate in human law. If the claim is that same-sex marriage is a right originating in divine law, then it must be false, since (if it is addressed at all) it is precluded by the holy books of all major religions. If the claim is that same-sex marriage is a right originating in human law, then it must, again, be false, since the law of the land (at least in the United States) does not acknowledge it. Therefore, in either case—whether the appeal is to divine or human law—the claim is false.
Now there are complicating factors here. The supporter of same-sex marriage could support his case by saying that it is a right originating in human law and that it can be found in the Constitution itself (either a state constitution or the federal Constitution). In fact, this is exactly the argument some make. The weakness of that position, however, is that they would then commit themselves to abandoning their belief in this right as soon as the constitution in question is amended. At that point, their only refuge would be in some sort of metaphysical, God-given right, which is a much harder assertion to establish--particularly for people who, like many of those who take this position, either do not believe in metaphysics or are not terrible good at it.
There are many ways to attack the truth of an opponent’s premise. The above example is just one way to do it. Let’s look now at how to attack the inferences.
Attacking the inferences of an argument involves taking the logic of your opponent’s argument and applying it to something else. The object here is to use your opponent’s logic to produce a conclusion that even he will find unacceptable. This procedure is called reductio ad absurdum, which is Latin for “reduction to absurdity.” It is a very concrete way of showing how ridiculous your opponent’s argument is.
Let’s try this on another argument. Recently, the issue of Intelligent Design has been the subject of a vigorous national debate. Its proponents claim that there is a way to scientifically prove that certain things in the universe—or the universe as a whole—are the product of design. Its opponents charge that Intelligent Design is not science:
In order for a theory to be scientific it must be falsifiableIn appealing to falsifiability, the opponents of Intelligent Design are employing the criterion for science laid down by philosopher Karl Popper (who said that science, broadly speaking, is whatever holds itself out for potential falsification). If someone claims that there is a law of gravity, he climbs to the top of a building and drops things over the side and they fall to the ground. And every time he does this the same thing happens. The theory is scientific because it could be shown false by something being dropped from the top of the building and not falling to the ground. In this way, the law of gravity is falsifiable.
Intelligent Design is not falsifiable
Therefore, Intelligent Design is not science
The debate now is whether Intelligent Design is falsifiable in the same way. We could attack one of the assumptions here (like we attacked the assumption of the previous argument about same-sex marriage) by questioning the second premise. But let’s try attacking the inferences.
How would we do this? We would simply ask, if we accept the argument that Intelligent Design is not science because it is not falsifiable, then what else does not qualify as science? The idea here is to come up with something your opponent would say is science that is not science by the criteria he has laid out. It is to show him his logic is wrong because, if he applies it consistently, he will have to accept a conclusion he doesn’t want to accept. Is there one?
The best place to look for scientific theories that are not falsifiable is physics. Everyone accepts that physics, and the theories that are included under it, are scientific. But many of them are not falsifiable—at least not now. The most famous of these is superstring theory. Superstring theory is the theory that particles and fundamental forces in the universe can be explained by the vibration of very tiny symmetrical strings. The problem is that the theory is not only not falsifiable, but, as some scientists have pointed out, it isn’t even conceivably falsifiable. Some of Einstein’s thought experiments (many of which he later set forth as full scientific theories), the scientific status of which have never been questioned, are not falsifiable either.
Your opponent could swallow hard and say that these things are not science, but he will know he is on shaky ground—and he will know you know he knows it.
This simple principle, this logical shortcut, does not replace logic, and, in fact, can be performed much better and more easily if you know logic. But it is still useful to both the experienced logic student and the person with little formal logical training.