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HOW TO MAKE A GOOD DEDUCTIVE ARGUMENT

Gabriel Wilensky

I f you want to be effective at assessing and defending arguments, solving problems, and making effective decisions you must become adept at thinking logically. This is because our brains are highly flawed as a tool for understanding the world we live in, yet we are able to reason and we can and should use that reasoning ability to override the limitations of our cognitive abilities.

In the field of logic there are two methods of study: formal and informal logic. Here we’ll focus on informal logic. Formal logic is mathematical or symbolic and includes things like truth tables, and is great to increase your critical thinking and reasoning skills. But we’ll focus on informal logic as that will give you easier tools to help you deal with issues you are faced with on your daily life.

This is one of a series of articles in which we’ll cover how to make good arguments. Here we’ll take a close look at deductive arguments, and explore what they are, how they work and why it’s important you know all this. The ultimate goal is to lean how to recognize and how to make a good deductive argument.

What is a deductive argument?

A good deductive argument is one which supports its claims. In this type of reasoning we move from a conclusion to the premises that may provide evidence for it. We must evaluate whether the evidence for that conclusion is valid. This is because generally if the evidence is valid, so is the conclusion.

When you are presented with an argument and you want to evaluate whether it’s a sound one, it behooves you to first identify the type of argument it is, namely a deductive or an inductive argument, and then figure out whether the conclusion is logical.

Defining Terms

Claim: A statement about the value, truth or existence of something.

Argument: A claim supported by evidence.

Conclusion: The main claim or assertion in an argument.

Premises: Statements of evidence supporting the conclusion.

The conclusion is the main claim the argument is making; in essence, what the point of the argument is.

Breaking down a deductive argument into its components

Just like with any other problem, when faced with a deductive argument we must first break it down into its component parts. You might want to review the article 5 strategies to effectively solve problems where you can see how this is done. This process allows us to deal with more manageable pieces. Deductive arguments are no different in this regard, and thus our first step when we must evaluate an argument is to identify its parts:

  • The premises
  • The conclusion

The first and most important thing we need to identify is the conclusion. The conclusion is the main claim the argument is making; in essence, what the point of the argument is.

The premises are the pieces of evidence that support the conclusion. The premises are what give validity to the claim.

The difficulty in identifying the conclusion is that often it’s hard to tell the conclusion from the premises and vice versa. Moreover, in a deductive argument the conclusion can be anywhere in the argument and not—as you might expect—at the end.

A simple way to help you identify the conclusion of a deductive argument is to establish the main claim or point the argument is trying to prove. So, the first thing you should do is ask yourself what is the main idea the argument is trying to convince you of.

A simple way to help you identify the conclusion of a deductive argument is to establish the main claim or point the argument is trying to prove.

Identifying the conclusion

Let’s go through an example of how to parse a deductive argument to identify its conclusion. Consider this:

He was a Spartan, so he must have been a good warrior. All Spartans were good warriors.

Going through the process outlined above let’s break this argument down into its constituent parts:

  • He was a Spartan.
  • He must have been a good warrior.
  • All Spartans were good warriors.

As we discussed, the first thing you should do is identify the main point the argument is making. To do this it helps to see which premises have support (i.e. evidence) for them. Looking at the three claims above we can see that there is no support for the claims, “He was a Spartan” and “All Spartans were warriors”. However, there is evidence for the claim, “He must have been a good warrior”. And what is that evidence? How can we infer he must have been a good warrior? We can conclude this because we know he was a Spartan and that all Spartans were good warriors. Thus we can identify the conclusion as “He must have been a good warrior”.

Moreover, another clue that you can use is the term so or similar. Phrases or words like this are indicators that what follows is the conclusion.
INDICATE A CONCLUSION INDICATE A PREMISE
Accordingly As indicated by
As a result As shown by
Consequently Because
Hence For
It follows that Given that
So Inasmuch as
That's why Since
Therefore The reason is that
This means/suggests/shows that
Thus
You must remember that in a deductive argument the conclusion can be anywhere, so you can’t just look at the end of the argument and assume the conclusion will be there. For instance, the argument above can easily be restated as:

All Spartans were good warriors. He must have been a good warrior because he was a Spartan.

More often than not, in complex deductive arguments the conclusion will be stated at the beginning, before any of the supporting premises. For you, the key task is to identify what the main point of the argument is, as that will be the conclusion.

A good test to make sure you’ve chosen the right claim as the conclusion is to put the term because between it and the other premises.

Thus, in our example the test will look like this:

He must have been a good warrior because he was a Spartan and because all Spartans were good warriors.

If one or more of the premises in an argument is false then the argument based on that premise will not be sound.

How to evaluate an argument

There are two steps you must take to evaluate whether an argument you are presented with, or one you are constructing yourself, is sound.

First, you need to identify the premises in the argument and you will need to evaluate whether they are true or false. If one or more of the premises in an argument is false then the argument based on that premise will not be sound. This is the reason why it’s so important to identify and evaluate the premises of an argument, a step most people fail to do when examining an argument.

Second, you must ascertain whether the conclusion of the argument is logical. You may do this by asking:

  1. What is the conclusion?
  2. What is the evidence to support it?
  3. Is that evidence logical?

If the evidence supporting a conclusion is logical then the conclusion should also be logical and the argument sound.

For example, we may say, “All men are mortal. Joe is a man. Therefore, Joe is mortal.” Here we have:

Premise 1: All men are mortal.
Premise 2: Joe is a man.
Conclusion: Joe is mortal.

This is a valid and sound argument because the two premises are true and logical, and the logic of the argument works (i.e., we can infer the conclusion from the two premises).

If the logic of the argument works, then the argument is valid. Thus, whether an argument is valid or not is dependent on its form or structure.

Please note however that an argument may be valid and yet be false, since in a valid argument one or more of the premises may be wrong or false.

For example, an argument such as, “All men are grandfathers. Jim is a man. Therefore, Jim is a grandfather,” is logically valid but it is untrue because the original premise is false (it’s not true all men are grandfathers). The argument is not sound.

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It’s important to point out when evaluating the evidence for a conclusion (i.e. the premises) that we may not be able to establish with metaphysical certitude whether they are true. Thus, we must usually treat these statements as tentative, a consequence of our incomplete level of knowledge. To compensate for this limitation we must be content when things have been established to a sufficient degree that for all practical purposes they may be treated as fact.
What we are looking for is to construct or ensure the arguments we are presented with are sound arguments, and for an argument to be sound both its premises and its logic must be valid. So, if you are presented with an argument and you are able to break it down into its components and ascertain whether all of its premises are true and that its logic is valid, then you may conclude the argument is sound. If the premises in a sound argument are true, then its conclusion must also be true. Therefore, the conclusion of a sound argument also has to be true.

For an argument to be sound both its premises and its logic must be valid.

There’s one scenario you must be careful about, and that is assuming you can infer that the conclusion of an unsound or invalid argument is false. If you are able to demonstrate that an argument is unsound or invalid the only thing you can do is discard that argument as a justification for that conclusion. Determining that an argument is unsound or invalid does not in itself prove that the conclusion is false
To summarize, in a deductive argument we go from a general conclusion to specific evidence that supports that conclusion. The conclusion is the main claim of the argument, and the supporting claims are called the premises.

An argument will be valid when the logic in it works, in other words, when the conclusion can be logically derived from the premises. An argument may be valid, and yet it may be false if one or more of the premises are false. An argument is sound if both its premises and its logic are valid.

When you are presented with a deductive argument you want to ensure that the argument is sound and know that it makes sense.

To accomplish this follow the following steps:

  1. Identify its main conclusion
  2. Identify the premises that support the conclusion
  3. Determine if the premises are true
  4. Decide whether the logic of the argument is valid

If the premises are true and the logic is valid, then the argument is sound. This may sound like a lot of work, but if you train your mind to parse arguments this way every time you’ll be able to do it very quickly and eventually reflexively. This is the ultimate goal as this is one of the things that makes you a better thinker.

What do you think?

Share your thoughts with the Thought Academy community in the Comments section below.

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