Validity, Soundness, and Valid Patterns

Site: Saylor Academy
Course: PHIL102: Introduction to Critical Thinking and Logic
Book: Validity, Soundness, and Valid Patterns
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Date: Wednesday, October 20, 2021, 12:59 PM


Validity and soundness are two of the most important concepts in the study of arguments, and they are often confused with one another. Read these three tutorials, starting with A03 and clicking through to A05, on the distinction between valid and sound arguments, their relationship to the truth of the statements that make them up, and the structural patterns that help us to recognize them.

Complete the exercises and check your answers.


§1. Definition of Validity

One desirable feature of arguments is that the conclusion should follow from the premises. But what does it mean? Consider these two arguments:

    • Argument #1: Barbie is more than 90 years old. So Barbie is more than 20 years old.
    • Argument #2: Barbie is more than 20 years old. So Barbie is more than 90 years old.

Intuitively, the conclusion of the first argument follows from the premise, whereas the conclusion of the second argument does not follow from its premise. But how should we explain the difference between the two arguments more precisely? Here is a thought: In the first argument, if the premise is true, then the conclusion cannot be false. On the other hand, even if the premise in the second argument is true, there is no guarantee that the conclusion must also be true. For example, Barbie could be 30 years old.

So we shall make use of this idea to define the notion of a deductively valid argument, or valid argument, as follows:

An argument is valid if and only if there is no logically possible situation where all the premises are true and the conclusion is false at the same time.

The idea of validity provides a more precise explication of what it means for a conclusion to follow from the premises. Applying this definition, we can see that the first argument above is valid, since there is no possible situation where Barbie can be over 90, but not over 20. The second argument is not valid because there are plenty of possible situations where the premise is true but the conclusion is false. Consider a situation where Barbie is 25, or one where she is 85. The fact that these situations are possible is enough to show that the argument is not valid, or invalid.

§2. Validity and Truth

What if we have an argument with more than one premise? Here is an example:

  • All pigs can fly. Anything that can fly can swim. So all pigs can swim.

Although the two premises of this argument are false, this is actually a valid argument. To evaluate its validity, ask yourself whether it is possible to come up with a situation where all the premises are true and the conclusion is false. (We are not asking whether there is a situation where the premises and the conclusion are all true.) Of course, the answer is "no." If pigs can fly, and if anything that can fly can also swim, then it must be the case that all pigs can swim.

So this example tells us something :

The premises and the conclusion of a valid argument can all be false.

Hopefully, you now realize that validity is not about the actual truth or falsity of the premises or the conclusion. Validity is about the logical connection between the premises and the conclusion. A valid argument is one where the truth of the premises guarantees the truth of the conclusion, but validity does not guarantee that the premises are in fact true. All that validity tells us is that if the premises are true, the conclusion must also be true.

§3. Showing that an argument is invalid

Now consider this argument :

  • Adam loves Beth. Beth loves Cathy. So Adam loves Cathy.

This argument is not valid, for it is possible that the premises are true and yet the conclusion is false. Perhaps Adam loves Beth, but does not want Beth to love anyone else. So Adam actually hates Cathy. The mere possibility of this situation is enough to show the argument is not valid.

Let's call these situations invalidating counterexamples to the argument. Basically, we are defining a valid argument as an argument with no possible invalidating counterexamples. To sharpen your skills in evaluating arguments, it is therefore important that you are able to discover and construct such examples.

Notice that a counterexample does not need to be real in the sense of being an actual situation. It may turn out that Adam, Beth and Cathy are members of the same family and they love each other. But the above argument is still invalid since the counterexample constructed is a possible situation, even if it is not actually real. All that is required of a counterexample is that the situation is a coherent one in which all the premises of the argument are true and the conclusion is false.

So remember this:

An argument can be invalid even if the conclusion and the premises are all actually true.

Here is another invalid argument with a true premise and a true conclusion:

  • "Paris is the capital of France. So Rome is the capital of Italy." It is not valid because it is possible for Italy to change its capital (say to Milan), while Paris remains the capital of France.

Another point to remember is that it is possible for a valid argument to have a true conclusion even when all its premises are false.

Here is an example:

  • All pigs are purple in color. Anything that is purple is an animal. So all pigs are animals.

Before proceeding any further, please make sure you understand why these claims are true and can give examples of such cases.

  1. The premises and the conclusion of an invalid argument can all be true.
  2. A valid argument should not be defined as an argument with true premises and a true conclusion.
  3. The premises and the conclusion of a valid argument can all be false.
  4. A valid argument with false premises can still have a true conclusion.

§4. A Reminder

The concept of validity provides a more precise explication of what it is for a conclusion to follow from the premises. Since this is one of the most important concepts in this course, you should make sure you fully understand the definition.

When we give our definition, we are making a distinction between truth and validity. In ordinary usage "valid" is often used interchangeably with "true" (similarly with "false" and "not valid"). But here validity is restricted to arguments and not statements, and truth is a property of statements, but not arguments. So never say things like "this statement is valid" or "that argument is true"!

Exercise #1

Are these arguments valid? Click each box for the answer.

Source: Joe Lau and Jonathan Chan,
Creative Commons License This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License.


It should be obvious by now that validity is about the logical connection between the premises and the conclusion. When we are told that an argument is valid, this is not enough to tell us anything about the actual truth or falsity of the premises or the conclusion. All we know is that there is a logical connection between them, that the premises entail the conclusion.

So even if we are given a valid argument, we still need to be careful before accepting the conclusion, since a valid argument might contain a false conclusion. What we need to check further is of course whether the premises are true. If an argument is valid, and all the premises are true, then it is called a sound argument. Of course, it follows from such a definition that a sound argument must also have a true conclusion. In a valid argument, if the premises are true, then the conclusion cannot be false, since by definition it is impossible for a valid argument to have true premises and a false conclusion in the same situation. So given that a sound argument is valid and has true premises, its conclusion must also be true. So if you have determined that an argument is indeed sound, you can certainly accept the conclusion.

An argument that is not sound is an unsound argument. If an argument is unsound, it might be that it is invalid, or maybe it has at least one false premise, or both.

Exercise #1.
Is it possible to have arguments of the following kinds? It is particularly important to note the highlighted cases. Click on each box for the answer.

Exercise #2
Are the following statements true or false?

Valid patterns

With valid arguments, it is impossible to have a false conclusion if the premises are all true. Obviously valid arguments play a very important role in reasoning, because if we start with true assumptions, and use only valid arguments to establish new conclusions, then our conclusions must also be true. But which are the rules we should use to decide whether an argument is valid or not? This is where formal logic comes in. By using special symbols we can describe patterns of valid argument, and formulate rules for evaluating the validity of an argument.

§1. Modus ponens

Consider the following arguments :

      • If this object is made of copper, it will conduct electricity. This object is made of copper, so it will conduct electricity.
      • If there is no largest prime number, then 510511 is not the largest prime number. There is no largest prime number. Therefore 510511 is not the largest prime number.
      • If Lam is a Buddhist then he should not eat pork. Lam is a Buddhist. Therefore Lam should not eat pork.

These three arguments are of course valid. Furthermore you probably notice that they are very similar to each other. What is common between them is that they have the same structure or form:

Modus ponens - If P then Q. P. Therefore, Q

Here, the letters P and Q are called sentence letters. They are used to translate or represent statements. By replacing P and Q with appropriate sentences, we can generate the original three valid arguments. This shows that the three arguments have a common form. It is also in virtue of this form that the arguments are valid, for we can see that any argument of the same form is a valid argument. Because this particular pattern of argument is quite common, it has been given a name. It is known as modus ponens.

However, don't confuse modus ponens with the following form of argument, which is not valid!

Affirming the consequent - If P then Q. Q. Therefore, P.

Giving arguments of this form is a fallacy - making a mistake of reasoning. This particular mistake is known as affirming the consequent.

If Jane lives in Beijing, then Jane lives in China. Jane lives in China. Therefore Jane lives in Beijing. (Not valid. Perhaps Jane lives in Shanghai.)

There are of course many other patterns of valid argument. Now we shall introduce a few more patterns which are often used in reasoning.

§2. Modus tollens

Modus tollens - If P then Q. Not-Q. Therefore, not-P.

Here, "not-Q" simply means the denial of Q. So if Q means "Today is hot.", then "not-Q" can be used to translate "It is not the case that today is hot", or "Today is not hot."

If Betty is on the plane, she will be in the A1 seat. But Betty is not in the A1 seat. So she is not on the plane.

But do distinguish modus tollens from the following fallacious pattern of argument :

Denying the antecedent - If P then Q, not-P. Therefore, not-Q.

If Elsie is competent, she will get an important job. But Elsie is not competent. So she will not get an important job. Not valid. Perhaps Elsie is not very competent, but her boss couldn't find anyone else to do the job.

§3. Hypothetical syllogism

Hypothetical syllogism - If P then Q, If Q then R. Therefore, if P then R.

If God created the universe then the universe will be perfect. If the universe is perfect then there will be no evil. So if God created the universe there will be no evil.

§4. Disjunctive syllogism

P or Q. Not-P. Therefore, Q. ; P or Q. Not-Q. Therefore, P.

Either the government brings about more sensible educational reforms, or the only good schools left will be private ones for rich kids. The government is not going to carry out sensible educational reforms. So the only good schools left will be private ones for rich kids.

§5. Dilemma

P or Q. If P then R. If Q then S. Therefore, R or S.

When R is the same as S, we have a simpler form :

P or Q. If P then R. If Q then R. Therefore, R.


Either we increase the tax rate or we don't. If we do, the people will be unhappy. If we don't, the people will also be unhappy. (Because the government will not have enough money to provide for public services.) So the people are going to be unhappy anyway.

§6. Arguing by Reductio ad Absurdum

The Latin name here simply means "reduced to absurdity". Here is the method of argument if you want to prove that a certain statement S is false:

  1. First assume that S is true.
  2. From the assumption that it is true, prove that it would lead to a contradiction or some other claim that is false or absurd.
  3. Conclude that S must be false.

Those of you who can spot connections quickly might notice that this is none other than an application of modus tollens. A famous application of this pattern of argument is Euclid's proof that there is no largest prime number. A prime number is any positive integer greater than 1 that is wholly divisible only by 1 and by itself, e.g. 2, 3, 5, 7, 11, 13, 17, etc.

  1. Assume that there are only n prime numbers, where n is a finite number : P1 < P2 < ... < Pn.
  2. Define a number Q that is 1 plus the product of all primes, i.e. Q = 1 + ( P1 x P2 x ... x Pn).
  3. Q is of course larger than Pn.
  4. But Q has to be a prime number also, because (a) when it is divided by any prime number it always leave a remainder of 1, and (b) if it is not divisible by an prime number it cannot be divisible by any non-prime numbers either.
  5. So Q is a prime number larger than the largest prime number.
  6. But this is a contradiction, so the original assumption that there is a finite number of prime numbers must be wrong.
  7. So there must be infinitely many primes.

Let us look at two more examples of reductio:

  • Suppose someone were to claim that nothing is true or false. We can show that this must be false as follows : If this person's claim is indeed correct, then there is at least one thing that is true, namely the claim that the person is making. So it can't be that nothing is true or false. So his statement must be false.
  • One theory of how the universe came about is that it developed from a vacuum state in the infinite past. Stephen Hawking thinks that this is false. Here is his argument : in order for the universe to develop from a vacuum state, the vacuum state must have been unstable. (If the vacuum state were a stable one, nothing would come out of it.) But if it was unstable, it would not be a vacuum state, and it would not have lasted an infinite time before becoming unstable.

§7. Other Patterns

There are of course many other patterns of deductively valid arguments. One way to construct more patterns is to combine the ones that we have looked at earlier. For example, we can combine two cases of hypothetical syllogism to obtain the following argument:

If P then Q. If Q then R. If R then S. Therefore if P then S.

There are also a few other simple but also valid patterns which we have not mentioned:

  • P and Q. Therefore Q.
  • P. Therefore P.

Some of you might be surprised to find out that "P. Therefore P." is valid. But think about it carefully - if the conclusion is also a premise, then the conclusion obviously follows from the premise! Of course, this tells us that not all valid arguments are good arguments. How these two concepts are connected is a topic we shall discuss later on.

We shall look at a few more complicated patterns of valid arguments in another tutorial. It is understandable that you might not remember all the names of these patterns. But what is important is that you can recognize these argument patterns when you come across them in everyday life, and would not confuse them with patterns of invalid arguments that look similar.

Exercise #1

Consider the following arguments. Identify the forms of all valid arguments. Here are your choices: modus ponens, modus tollens, hypothetical syllogism, disjunctive syllogism, dilemma, reductio ad absurdum, valid but not one of the above patterns, invalid.

Exercise #2