Evaluating an Argument

Contact: Dr. Jan Garrett

Last revised date: September 30, 2006

1. Informal Fallacies
2. Deductive or Inductive?
3. Detecting Fallacies in Inductive Arguments
4. Validity and Invalidity in Deductive Arguments
5. Challenging Valid and Strong Arguments
6. Weak Critique: Common Errors in Responding to Arguments
7. The Value of Internal Criticism
8. The Value of Knowing Alternate Views

For some students, this will mostly be a review and a reminder. (Reminders can be beneficial only if they affect action!)

Probably the best way to learn how to evaluate arguments is to take a course in logic and while in the course apply yourself to learn the skills as if your life is going to depend upon it. (If it is true that the unexamined life is not worth living, then something more important than mere life depends upon it, namely, the well-being of your mind!)

Since it is impossible for a brief web page to substitute for a logic course, all I can do here is make suggestions, give reminders, and perhaps suggest some further reading.

Once you have broken up the more complex argument you wish to evaluate into its component "simple" arguments (see Argument Analysis (2006 Version)), you can ask certain pointed questions about the "simple" arguments that make up the more complex one.

Informal Fallacies

Sometimes you can discover one of the many informal fallacies that occur so often they have been given names and are discussed in most introductory logic textbooks and in chapters on Logic in introductory philosophy textbooks. These fall into a few main categories, such as Fallacies of Relevance (such as Bandwagon, Illegitimate Appeal to Authority, Ad Hominem, Illegitimate Appeal to Pity, etc.) and Fallacies of Ambiguity (Equivocation, Amphiboly, Composition, Division).

The better philosophers are aware of these types of fallacy and make special effort to avoid them. Such fallacies occur more often in informal argumentation, in political debate, and in advertising. (This does not mean that philosophers are entirely immune to them.)

Deductive or Inductive?

You'll probably need to decide whether the author intends the argument to be a deductive or an inductive argument. Does the author intend the premises to imply the conclusion with necessity? (If yes, treat the argument as deductive.) Or does the author intend the premises to make the conclusion probable? (If yes, treat the argument as inductive.)

Inductive Arguments

If the argument is an inductive argument, the general question you should ask is whether the premises, assuming their truth, give the degree of probable support to the conclusion that the author appears to claim? (The premises might give a little support to the conclusion but not as much as the author wants us to believe.)

If the inductive argument is causal argument, watch out for common fallacies of causal reasoning, such as After This Therefore Because of This.

If the inductive argument is trying to reason from a fact, supposed to be an effect of something else, to its alleged cause, we are probably faced with an argument type known as The Argument to the Best Explanation. But what seems to be an explanation of a fact (or something supposed to be a fact for purposes of evaluation) may explain the fact without being the best explanation available. If there are better explanations, the particular argument from effect to cause may be weak.

If the inductive argument draws a general conclusion from premises about particulars, watch out for the Fallacy of Hasty Generalization.

Another style of inductive argument is the argument from analogy: Analogical arguments can be weak in various ways. (See a logic text for details.)

Deductive Arguments

Check for validity. A deductive argument is valid just in case if the premises were true, then the conclusion must be true. Otherwise, the deductive argument is invalid.

Formal validity. Sometimes the validity of an argument results from its logical form: for instance, categorical syllogisms that have the following form are valid:

All M are P
All S are M
:.All S are P

In the symbolic form of categorical syllogisms, the letters stand for "categories" or general descriptions (typically designated by nouns or noun phrases). Thus "S" might stand for humans, "M" might stand for mammals, and "P" for mortal beings.

And propositional syllogisms of the following form are valid:
If p, then q

In the symbolic form of such syllogisms, then letters stand for statements (also called "propositions"). Thus "p" and "q" might stand for "it is raining" and "the ground is wet," respectively.

Other patterns may be valid or invalid depending on the precise logical pattern. Introductory logic textbooks and courses will tell you which patterns are which, or give you tools to determine validity or invalidity for yourself. The most common valid and invalid patterns have been given names to help you remember. The valid categorical syllogism form above is called Barbara (also AAA-1; see a Logic text for explanation of this notation). The valid propositional syllogism form given above is Modus Ponens.

The following deductive forms are two of many invalid forms:

All P are M
All S are M
:.All S are P

If p, then q

Arguments that have the latter form are said to commit the Fallacy of Affirming the Consequent.

How Can I Challenge a Valid or Strong Argument?

A valid argument is a deductive argument the truth of whose premises would necessitate the truth of the conclusion. A strong argument is an inductive argument the truth of whose premises would give as much probable support to the conclusion as the author claims.

Such arguments can still be flawed if their premises are false. So they can be challenged by good arguments that show the falsity of one or more of their premises.

One can sometimes construct inductive arguments for conclusions to the effect that a premise in another person's argument is false. (This sort of reasoning occurs in historical or scientific research, investigative journalism, detective work, legal argumentation, moral reasoning, and political advocacy.)

If the premise is a general statement of the form "All S are P" or "No S are P" (or various parallel forms such as "Whenever S occurs, P occurs," or "Whenever S occurs, P does not occur"), you can disprove it by discovering a counter-example. "All S are P," for instance, is disproven by the discovery of an S that is not a P.

If the premise is meant as a definition, and one based upon the ordinary meaning of the term, you can challenge the definition by producing a counter-example. ("A bachelor is an adult unmarried male" is refuted by mentioning a male cat, whom we would never call literally a bachelor. But this counter-example would be ineffective against "A bachelor is an adult unmarried human male.")

Weak Critique: Common Errors in Responding to Arguments

1. Unsupported denial of the conclusion.

Plato argues that the soul exists before a human being is conceived. But this is false.
This method of "critique" fails to give any reasons for denying Plato's view. It does not really count as critique at all.

2. Appeal to mere opinion or feeling.

Plato argues that the soul exists before a human being is conceived. But in my opinion this is false.
The mere fact that someone believes a proposition about something outside his or her own consciousness to be true or false is no evidence that it is. This method of "critique" too does not really count as critique.

3. Denial of conclusion supported by a reason whose significance for the position of the author being criticized is unexamined.

Plato argues that there are intelligible forms, such as Beauty, Justice, and Equality. But this is false. Nobody has ever seen such things.
The author of this "refutation" seems completely unaware that Plato himself admits that nobody has ever seen such things, i.e., with the eyes, but Plato distinguishes between seeing with the eyes and mentally "seeing" (i.e., conceiving or understanding) "with the mind" or rational part of the soul. The author of this "critique" fails to realize how easy it would be for Plato or a Platonist to undermine the criticism.

The Case for Internal Criticism

Internal criticism does not try to directly refute the author's conclusion. Rather it shows that on the premises of the author herself, she should or could have drawn a different conclusion. Perhaps she is using an unstated assumption that is false or possibly false and, if a true or more probably true assumption were substituted for it, the author's conclusion would not follow and a different conclusion could be proven.

Internal criticism does not rule out developing an alternate argument, with several new premises (constituting a rival theory), whose conclusion contradicts the conclusion of the author under criticism. Internal criticism can prepare the way for development of a new, rival theory, or if the rival theory already exists, for appreciation of that theory's strengths.

The Importance of Being Familiar with Alternate Views
and Thus of the History of Philosophy

and Earlier Contributions to Moral Debates

Today's philosophers and philosophy students do not have to reinvent the wheel when they confront a superficially persuasive but flawed positions. (A parallel point can be made for today's moral reasoners and ethics students.) These positions have probably been around for some time and have attracted earlier critics. In evaluating an argument it helps to know about the thinkers who critically evaluated it in its earlier incarnation. One can often use the ideas of the earlier critics, or adapt them for use today. (The example that follows concerns a metaphysical and epistemological debate.)

For instance, in his dialogue The Phaedo Plato tries to show that the soul is separable from the body. He intends to prove later that it is immortal. He argues for his conclusion from premises that
(1) we have general notions (say, of equality and beauty);

(2) we could not have gotten them from sense experience;

(3) the only experience we have had since being in our present bodies is sense experience;

(4) for any notion we have, we either got it from sense experience or independent of sense experience when we were free from our bodies.

Aristotle and others following him try to show, among other things, that (2) is false: we can gain general ideas by abstracting the intelligible form from particulars that we experience and that consist of matter and form. I can acquire the general notion of Sphere by abstracting it from, say, balls and other spherical material objects. Once I have learned what a sphere essentially is, I have in my mind the notion of Sphere that corresponds to the spherical form in such external objects. (This is a simplified account of Aristotle's theory of how we learn general ideas, but it indicates a strategy that one might use to refute the Platonic approach.)
Thus in evaluating Plato's claim it certainly helps to be aware of Aristotle's.

The example of Aristotle's response to Plato also illustrates the importance of philosophical creativity. At the time Plato argued that the soul existed in an out-of-body state prior to this life, he did not seem to have among his philosophical tools the general theory of forms inside matter from which something like intelligible copies could be abstracted and "stored," so to speak, as mental notions. Apparently Aristotle himself created this conceptual tool, partly in response to his dissatisfaction with aspects of Plato's philosophy. Thanks to Aristotle, we can show how humans acquire general notions even if their souls have not ever existed in a prior out-of-body state. We simultaneously show that the argument (attributed to Plato) above for the separability of the soul does not rest upon premises immune to serious challenge.