20 Arguments V: Introduction to Deductive and Inductive Arguments

An argument, as we are using the term, is a series of claims (the premises) which attempt to establish the truth or probable truth of another claim (the conclusion). The premises thus give reasons someone is supposed to believe that the conclusion is true. Or, put differently, the conclusion is inferred from the premises.

We’ve focused on identifying what is left out or unstated in arguments (hidden or implicit premises). And we’ve talked about the need to tease out different strands of reasoning for a given conclusion, by identifying overlapping arguments that are each made up of sets of inter-connected premises.

But we’ve otherwise been lumping all arguments together. Since our goal is to be able to assess arguments, i.e., to see when they provide good reasons for a conclusion, we now need to make a distinction between two different kinds of arguments. This will let us see that there are different standards for judging the success of an argument, and common kinds of errors, or “fallacies” that go with each. (While we’ll cover a couple of these fallacies in this Module, it’s really only in the next that we will focus on them in detail.)

Two Kinds of Arguments

The distinction between kinds of arguments is rooted in the different intent they have: some arguments aim for a conclusion that follows necessarily from the premises. That is, if the premises are true, then the conclusion must be true as well. Other arguments, however — and, indeed, most of the arguments we actually make in ordinary life and even in science — only aim for a conclusion that is probably true, given the truth of the premises. That’s because most of our reasoning occurs in situations where there is some degree of uncertainty or ignorance.

Arguments with these different aims have different names.

• A deductive argument is an argument whose conclusion is intended to follow with logical necessity from the given premises.
• An inductive argument is an argument whose conclusion is intended to follow with some degree of probability from the given premises.

Here’s a quick example of each, just to illustrate the difference. Note that the conclusion is the same in each; what differs is how it is being supported by its premises.

In the first, deductive argument, the two premises necessarily imply or logically entail the conclusion. That is, you couldn’t assert them as true without also logically committing yourself to the truth of the conclusion, on pain of logical contradiction. (And remember back to our reading of Plato: a basic requirement for good reasoning is that it be coherent, i.e., non-self-contradictory.)

Note that whether the conclusion is logically implied by the premises has nothing to do with the truth of any of the claims involved. Even if the premises are false, a conclusion can still logically follow from them.

In the second, inductive argument, the two premises taken together give you some reason for thinking the conclusion will be true. A pattern of weather over a couple of days is a good, if imperfect, basis for predicting what the next day’s weather will be. But obviously, weather changes, so even if the prediction is likely to be true, it’s not necessarily true. Even if the premises are true, and even if they provide good reasons for thinking the conclusion will be true, it might still turn out that the conclusion is false.

This is characteristic of inductive arguments. The premises explain why you think a conclusion is likely to be true, but it builds in an acknowledgement that the conclusion might turn out to be false. Thus, it’s never an objection to an inductive argument that the conclusion might not be true. The question to ask is whether the premises give you reasons for thinking that the conclusion is more likely to be true than not to be.

Now, when reasons are given for a conclusion that is to some degree uncertain, as happens in inductive arguments, that often means the person giving the reasons is, in some way, making a judgment call about what’s likely. So, there is some individual assessment built into much inductive reasoning. However, that doesn’t mean that you’re dealing with mere opinion. An argument gives reasons that others are supposed to be able to accept. It can do this well or not. So, it’s irrelevant whether there is opinion involved. The question is whether the reasons given are compelling or not.

And relatedly:

Crucial Point: The fact that claims about what is probable or likely involve an element of individual judgment on the part of the person making them does not mean that the person’s values are involved.

For example, a meteorologist who says it is likely to rain tomorrow may disagree with another who says that it is not likely to rain. Even though this dispute involves a difference in individual opinion — professional opinion in this case — there is no issue about what’s good/bad/right/wrong/should/shouldn’t be done. This is a dispute purely about facts, not values.

The next two chapters will go into depth and detail about inductive and deductive arguments. Each will have some exercises associated with it for you to do to practice identifying and analyzing them.