What is Informal Logic? - Types and Examples of Arguments

Informal logic is concerned with our everyday activities of making and establishing claims, evaluating claims, and detecting errors in reasoning. Its primary objective is to enhance the habit of straight, clear, and correct reasoning. The essential features of this habit include the avoidance of fallacies and ambiguities and the promotion of effective communication.

Informal logic is that aspect of logic that assesses and analyzes arguments that occur in natural language, that is, every day ordinary language. It seeks to explain and evaluate the arguments found in discussions, debates, and disagreements as they are manifested in daily life. Topics that characterize the field of informal logic include the nature of arguments, identification of arguments, kinds of arguments, and fallacies.

Nature of Arguments

An argument is made up of statements or propositions that can be divided into two main components. At least one of the statements in an argument would constitute the premise, with at least another statement constituting the conclusion. The premise(s) and conclusion(s) must be related in such a way that the former provides support or ground for accepting the latter.


Put differently, the conclusion of an argument is the main claim, belief, or position that a person intends to put forward for acceptance and the premise(s), constitute(s), justification(s), reason(s), or ground(s) that one intends to use in supporting the claim or belief.

Examples of Arguments

Example 1: All men are mortal.

Aristotle is a man.

Therefore, Aristotle is mortal.

The first and second statements are the premises, while the third statement is the conclusion of the argument.


Example 2: All politicians are liars.

Adebayo is a politician, and he lies a lot.

Isiak is a politician and a known liar.

Jonathan is a politician and a liar.

The first statement is the conclusion, while the other three statements are the premises of the argument above.


Example 3: The government has decided to remove oil subsidies by 2012.

Therefore, the masses are bracing up for a mass protest against the subsidy removal.

People generally believe that this would further exacerbate the economic hardship in the country.


It should be noted that there is no given order that the premise(s) and conclusion of an argument must take. Any of the two can come first or last. In some cases, the conclusion can even be between the premises. In the case of Example 1 above, the conclusion is found in the last statement of the argument, while it is the first statement in Example 2. In the case of Example 3, the conclusion is found between the premises.

Identifying Arguments

In logic, it is usual and quite convenient to present arguments in a standard form such that the premises come first before the conclusion.


The conclusion is normally indicated with the therefore sign (:).


Example:

All lawyers are liars.

Aderemi is a lawyer.

Therefore, Aderemi is a liar.


However, in ordinary daily discourse, arguments are not usually stated in this standard form. They can be presented in diverse shapes and forms. Hence, there is a need to be able to recognize arguments when they occur in prose or in our regular discourse. It is also important that we can identify the premises and the conclusion of an argument. In this regard, we must know that certain words or phrases usually indicate the premise(s), and these are called premise indicators and conclusion indicators.

Premise Indicators

Premise indicators are terms, words, or phrases that point to or precede the premise(s) of an argument. Examples of these are: since; because; for; as; following from; in view of; as shown by; in as much as; otherwise; is substantiated by; as indicated by; the reason is that; for the reason that; may be inferred from; may be derived from; and in view of the fact that.

Conclusion Indicators

Conclusion indicators are terms, words, or phrases that announce the conclusion of an argument. Some of these are: therefore; hence; thus; it is the case that; in consequence; as a result; proves that; accordingly; it follows that; we may infer that; I conclude that; which shows that; which means that; which allows us to infer that; which implies; and which points to the conclusion that.

Types of Arguments

There are two types of arguments: inductive and deductive arguments.

1. Inductive Arguments

In an inductive argument, one reasons from particular premises to a universal conclusion. However, the premises of an inductive argument do not give conclusive or one hundred percent support to the conclusion. Rather, the premises of inductive arguments only give some support to the conclusion. This support may be strong, weak, or nil, but certainly not total. In an inductive argument, it is possible to accept the premises and deny the conclusion without any contradiction.

Examples of Inductive Arguments

Example 1: Awolowo is human and mortal.

Azikiwe is human and mortal.

Ojukwu is human and mortal.

Therefore, probably all humans are mortal.


Some inductive arguments involve reasoning from universal premises to a universal conclusion or from particular premises to a particular conclusion, as shown by examples 2 and 3 below, respectively.


Example 2: All goats are mammals and have hearts.

All dogs are mammals and have hearts.

All donkeys are mammals and have hearts.

Therefore, probably all mammals have hearts.


Example 3: Idi Amin was a dictator and was ruthless.

Abacha was a dictator and was ruthless. Castro is a dictator.

Therefore, Castro is probably ruthless.

2. Deductive Arguments

A deductive argument is one in which you move from universal premises to a particular conclusion. Also, the premises of a deductive argument provide a conclusive ground or 100% support for its conclusion in such a way that, if the premises are accepted, the conclusion cannot be rejected. 


However, sometimes deductive arguments are such that you argue from universal premises to a universal conclusion, as shown in example 2 below. Hence, we can say that a deductive argument is an argument in which the premises logically imply the conclusion. In a deductive argument, if the premises are true, the conclusion cannot be false.

Examples of Deductive Arguments

Example 1: All humans are mortal.

Awolowo is human.

Therefore, Awolowo is mortal.


Example 2: Every mammal reproduces.

All dogs are mammals.

Therefore, every dog can reproduce.

Essential Differences between a Deductive and an Inductive Argument

Deductive Arguments

  • If all the premises are true, the conclusion must necessarily be true.

  • All the information or factual contents of the conclusion are contained, at least implicitly, in the premises.

Inductive Arguments

  • If all the premises are true, the conclusion is probably, but not necessarily, true.

  • The conclusion usually goes beyond the premises and contains information not contained, even implicitly, in the premises.

Exercises

Let find out if the arguments below are deductive or inductive.


Q1: Sugar is a cube; therefore, it has six sides.


Answer: This is an inductive argument. While cubes generally have six sides, sugar cubes can be irregularly shaped or broken. The conclusion is based on a common association but isn't guaranteed.


Q2: The lights were out and the car was gone, so we concluded that nobody was home.


Answer: This is another inductive argument. The evidence (lights out, car gone) points towards no one being home, but it's not conclusive. Other possibilities exist (power outage, car parked elsewhere).


Q3: Jonathan said he would stay in Abuja or go to Yenagoa for Christmas. He didn't stay in Abuja, so he must have gone to Yenagoa.


Answer: This is also inductive. Jonathan might have gone somewhere else entirely, or the initial premise about his plans could be wrong. While the conclusion seems likely, it's not certain.


Q4: Every human breathes. John is human, so he breathes.


Answer: This is a deductive argument. It uses the general rule "Every human breathes" and applies it to a specific case (John) to reach a guaranteed conclusion (he breathes). There's no room for uncertainty or probability.


Q5: Everyone interested in the job must write an examination. Benson wrote the examination, so he probably was interested in the job.


Answer: This is inductive as well. While writing the exam suggests potential interest, it doesn't guarantee it. Benson could have had other reasons for taking the exam (practice, fulfilling coursework requirements).


Q6: Every horse that has ever been observed has had a heart; therefore, every horse has a heart.


Answer: While not the strongest example, this also falls under deduction. It relies on the assumption that all past observations will hold true for all future cases. However, depending on the context, such an assumption might be less reliable.


Q7: All egrets are white because all the egrets I have seen are white.


Answer: This is a classic example of inductive reasoning. It generalizes from a limited set of observations ("all egrets I've seen are white") to a broader claim ("all egrets are white"). This conclusion is not guaranteed and likely incorrect, as other egret species exist with different colors.

Truth, Validity, and Soundness

While truth and falsehood can only be assigned to propositions and not arguments, the attributes of validity and invalidity belong to only deductive arguments. In other words, it is only deductive arguments that can be said to be valid or invalid. However, there is an essential connection between the validity or invalidity of an argument and the truth or falsehood of its premises and conclusion.


An argument is valid if the premises logically imply the conclusion. A valid argument is one in which it is impossible to accept the premises and deny the conclusion without self-contradiction. The validity or invalidity of an argument is formal in the sense that it is the form of the argument that determines if it is valid or not, not the truth or falsity of its premises. This is because we can have a valid argument with false premises and conclusions.


Put differently, a deductive argument is valid only when the premises, if true, provide conclusive grounds for the conclusion. We say that a deductive argument is invalid only when its premises are true and the conclusion is false.


If an argument is valid, any argument that has that form will be valid. Similarly, if an argument is invalid, the argument that also has that form will be invalid. An argument is sound if it is valid, has true premises, and has a true conclusion. Unless an argument is valid, it cannot be sound.

Examples of Sound Arguments


Example 1: All goats are mammals.

All mammals have lungs.

Therefore, all goats have lungs.


Example 2: All humans breathe.

All Nigerians are human beings.

Therefore, all Nigerians breathe.


Example 3: All Ghanaians are Africans.

All Akans are Ghanaians.

Therefore, Akans are Africans.

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