Ethical Realism

July 8, 2013

Unstated Premises

Filed under: philosophy — JW Gray @ 5:18 am
Tags: , ,

I will discuss what unstated premises are, how to identify them, and how to determine what they are.

What are unstated premises?

Unstated premises are premises that a deductive argument requires, but are not explicitly stated. Deductive arguments are popular and can be rationally persuasive, but people don’t always state all of the premises that their deductive arguments require. These premises can be called “unstated premises,” “missing premises,” or “hidden assumptions.” For example, consider the following argument:

  1. Socrates is a human.
  2. Therefore, Socrates is mortal.

This argument might seem persuasive as it is stated here, but it would actually be logically invalid. (The form would be “A. Therefore, B.” This is logically invalid because it would be possible for arguments with this form to have true premises and a false conclusion at the same time.) The reason we might find it persuasive is because of our assumptions. The unstated premise might seem too obvious to even mention—that if Socrates is a human, then Socrates is mortal (or that all men are mortal). We could then write the argument along with that unstated premise as the following:

  1. Socrates is a human.
  2. If Socrates is a human, then Socrates is mortal.
  3. Therefore, Socrates is mortal.

Once the unstated premise is added, the argument is logically valid. It would be impossible for arguments with that form to have true premises and a false conclusion at the same time. (The form is “A. If A, B. Therefore, B.”)

Identifying unstated premises

Identifying unstated premises requires common sense and a charitable interpretation. It also requires us to determine if an argument is deductive or if it’s invalid. There is no mechanical process that can tell us when unstated premises are involved.

First, we need to know if an argument is inductive or deductive. If it’s inductive, then it probably doesn’t have an unstated premise. Inductive reasoning is based on generalizations and predictions. For example, we know that no dogs lay eggs (in part) because every dog that’s ever reproduced gave birth to live young. We predict that they will continue to give birth to live young rather than lay eggs in the future, and we generalize that our observations are true of all dogs. Inductive arguments are meant to tell us what is probably true assuming the premises are true, so they are not meant to be logically valid. (It is possible for a good inductive argument to have true premises an a false conclusion.)

If an argument is not a prediction or generalization, then it’s probably a deductive argument. In that case the conclusion should follow from the premises. The argument should be meant to be valid—whoever gives the argument should intend it to be impossible for the premises to be true and the conclusion false. Deductive arguments almost always have more than one premise. If a deductive argument only has one premise, then there’s generally an unstated premise. It is charitable to determine an argument to have an unstated premise when it will assure us that a deductive argument with only one stated premise to be valid.

For example, consider the following argument:

  1. If women have a right to their body, then abortion should be legal.
  2. Therefore, abortion should be legal.

There’s only one premise and one conclusion, and the argument is not a prediction or generalization. Here the unstated premise could be “women have a right to their body.” In that case the argument will be logically valid with the following two premises:

  1. If women have a right to their body, then abortion should be legal.
  2. Women have a right to their body.
  3. Therefore, abortion should be legal.

Second, we need to know if a deductive argument should be interpreted as being invalid. Even if we are being charitable, it might be appropriate to determine an argument to be logically invalid. If an argument is logically invalid, then it’s unlikely that there’s an unstated premise.

Consider someone who argues the following:

  1. If it’s wrong to hurt people without an overriding reason, then punching people without an overriding reason is wrong.
  2. Punching people without an overriding reason is wrong.
  3. Therefore, it’s wrong to hurt people without an overriding reason.

This argument is logically invalid and has the form “If A, then B. B. Therefore, A.” Another argument with this form, true premises, and a false conclusion is “If all dogs are reptiles, then all dogs are animals. All dogs are animals. Therefore, all dogs are reptiles.”

In this case it’s not clear how a hidden premise could help, and we could imagine someone finding this argument to be persuasive. As a result it seems reasonably charitable to interpret the argument as having no unstated premises.

Determining when an argument is logically invalid can be done using a mechanical process. (See What You Need From Propositional Logic.)

How to determine unstated premises

I have given examples of unstated premises, but how do we know what exactly the unstated premises are? Once more, some common sense and charitable interpretation is required. There is no mechanical process that can tell us what exactly the unstated premises are. Even so, I suggest trying to find unstated premises by using various valid argument forms as an outline:

Modus ponens

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

Modus tollens

  1. If A, then B.
  2. Not-B.
  3. Therefore, not-A.

Disjunctive syllogism

  1. Either A or B.
  2. Not-A.
  3. Therefore, B.

Hypothetical syllogism

  1. If A, then B.
  2. If B, then C.
  3. Therefore, if A, then C.

Constructive dilemma

  1. Either A or B.
  2. If A, then C.
  3. If B, then D.
  4. Therefore, either C or D.

We can try to keep these valid forms of argument in mind when we decide which premise is missing. The stated premises and conclusion can often be made valid by using a premise from one of these argument forms. Consider someone who argues the following:

  1. If killing is always wrong, then the death penalty is wrong.
  2. Therefore, the death penalty is wrong.

The argument has the form “If A, then B. Therefore, B.” The unstated premise could give this argument the form of modus ponens, and the unstated premise will then be “killing is always wrong.” In that case the argument will be the following:

  1. If killing is always wrong, then the death penalty is wrong.
  2. Killing is always wrong.
  3. Therefore, the death penalty is wrong.

Although this argument is valid, I believe it is still flawed. The fact that the argument is interpreted charitably does not guarantee that the premises are true. In this case I believe that the unstated premise is too extreme (that killing is always wrong). Perhaps killing is morally justified when necessary to save several lives.

Finally, consider someone who argues the following:

  1. Evolution is not the best explanation for our observations.
  2. Therefore, creationism is the best explanation for our observations.

If we add a hidden premise, then this argument could either use modus tollens or the disjunctive syllogism.

If the argument uses modus tollens, then the unstated premise is “if creationism is the best explanation for our observations, then evolution is the best explanation for our experiences” and the argument will become the following:

  1. If creationism is the best explanation for our observations, then evolution is the best explanation for our experiences.
  2. Evolution is not the best explanation for our observations.
  3. Therefore, creationism is the best explanation for our observations.

This argument doesn’t quite sound right. You wouldn’t think that creationism being the best explanation for our experiences would somehow assure us that evolution is the best explanation for our experiences.

If the argument uses the disjunctive syllogism then the unstated premise is “either evolution is the best explanation for our experiences or creationism is the best explanation for our experiences” and the argument will become the following:

  1. Either evolution is the best explanation for our experiences or creationism is the best explanation for our experiences.
  2. Evolution is not the best explanation for our observations.
  3. Therefore, creationism is the best explanation for our observations.

This time the interpretation of the argument and the unstated premise seems charitable. Even so, we might reject one or both of the premises.

Conclusion

Debate can become difficult when we don’t properly understand the arguments other people present, and identifying unstated premises is often necessary to properly interpret other people’s arguments. To identify when an argument has an unstated assumption requires us to know when it’s a valid argument, and when a deductive argument should be interpreted as being an invalid argument with no unstated premises. Moreover, we can often figure out what an unstated premise should be by understanding various types of valid argument forms. If we assume the argument is meant to be valid, then the unstated assumption can often be found by using the valid argument forms as a guide.

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1 Comment »

  1. Thank you for this 🙂

    Comment by Abrar Farhan Zaman — October 10, 2016 @ 8:45 pm | Reply


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