PHI 103 Informal Logic – Week 2 Discussion

In the required reading this week we learned about the following argument forms: Deductive arguments, statistical syllogisms, arguments from analogy, appeals to authority, and inductive generalizations.

Choose three of the five argument forms and create an example of each. Your arguments do not have to be great arguments (you will analyze their quality below), but they should clearly be instances of the argument types you chose in question (make sure to indicate what argument type each one is).

After each argument, provide a brief analysis of its quality. If it is deductive, is it valid? Is it sound? If it is inductive, is it strong? Is it cogent? (Be sure to read the definitions from the text.) What might be done to improve the argument? What can people do to better understand and apply this type of argument in general?

INSTRUCTOR EXAMPLE POST

Hello Everyone,

Here’s an example post of what this week’s discussion should look something like. The three argument types I’ve chosen to discuss are the argument by analogy, inductive generalization, and the categorical syllogism.

Let’s begin with the first argument form, the analogy. Our textbook explains how these arguments work in the following way: “An argument from analogy is an inductive argument that draws conclusions based on the use of analogy. An analogy is a comparison of two items.” (Hardy et al 5.7 2015) For example, analogy arguments should follow this form:

B is similar to A.
A has feature F.
Therefore, B probably also has feature F.

So, an argument by analogy might be something like this:

P1: My frog is like your frog.

P2: Your frog is green.

C: Therefore, my frog is green.

The strength of an analogy is increased proportionally given a greater number of shared properties between the analogs. In this case, noting whether the frogs are of the same kind, from the same store, etc.

Let’s look at the second form, the inductive generalization. Our textbook explains how this form works in the following way: “An inductive generalization is an argument in which we reason from data about a sample population to a claim about a large population that includes the sample.” (Hardy et al 5.3 2015) For example, the form should look like the following:

X% of observed Fs are Gs.
Therefore, X% of all Fs are Gs.

Returning to the frog example, an inductive generalization argument might look like the following:

P1: 80% of observed frogs are green.

P2: [Assumption about the population.]

C: Therefore, 80% of all frogs are green.

The strength of these types of arguments is dependent on the way that the observation about the first population relates to the wider population. The sample needs to be representative, and our textbook explains this further: “We say that a sample is representative of a population when the sample and the population both have the same distribution of the trait we are interested in—when the sample “looks like” the population for our purposes.” (Hardy et al 5.3 2015) Representativeness depends on factors like randomness, sample population size, and a margin of error. So long as these conditions are sufficient, our confidence level increases in the strength of the generalization.

Let’s turn to the third and last argument, the categorical syllogism. Our textbook explains this argument in the following way: “The categorical syllogism, in which a conclusion is derived from two categorical premises, is perhaps the most famous—and certainly one of the oldest—forms of deductive argument.” (Hardy et al 3.6 2015) The form uses two types of statements to connect or distribute three terms (major, minor, middle) in the right way in order to guarantee that the conclusion is true so long as the premises are true. Here’s what the form looks like:

All S are M.
All M are P.
Therefore, all S are P.

So, if we want to say something about frogs, we might form the following argument.

P1: All frogs are amphibians.

P2: All amphibians need water to survive.

C: Therefore, all frogs need water to survive.

Here, we can see that if the two premises are true, then the conclusion is necessarily true too.

For more discussion of these common argument forms, see the relevant sections of the textbook.

Reference

Hardy, J., Foster, C., & Zuniga y Postigo, G. (2015). With good reason: A guide to critical thinking. Retrieved from https://content.ashford.edu/