Logic and Software Testing


involved in bug advocacy, informal logic is invaluable to testers. It lets you argue why something is in fact a bug and why that bug is important enough to fix. In a broader context, informal logic is the tool that lets you advocate for any of the myriad ideas and initiatives that you come up with. It may be clear to you that the test team needs more resources or that one feature should be prioritized over another, but neither course of action is likely to happen unless you can present a convincing argument for it to someone in management.

Logical Fallacies: Learning by Counterexample

While studying the attributes of a sound argument seems like the most straightforward way to get a grasp of informal logic, learning about common logical fallacies is a more accessible route to take. The language used to describe them is colourful (as opposed to the rather dry theory often associated with formal logic), and you can usually come up with instances of their occurring in your life or workplace.

To illustrate, let’s see what specious arguments we can muster to rally support for a statement that is patently false. I’m going to make the claim that pigs can fly. When someone points out that they can’t, I can try to discredit him or her personally: “Do you have a degree in biology? And, didn’t you just tell a lie the other day?” This is called ad hominem (Latin, “to the man”). I’m attacking the person presenting the argument instead of refuting the argument’s substance. Alternatively, I can reply with the following: “Why do you think animals can't fly? Birds fly all the time.” This is a straw man . I’ve misrepresented my opponent’s argument, then attacked that misrepresentation instead of the actual argument.

When it comes to affirming my own dubious position, I have many options:

    • Argumentum ad populum (“appeal to the people”)—“Ten thousand people believe that pigs can fly, so surely they can.” This is claiming that something is true because many people think it is.
    • Argumentum ad verecundiam (“appeal to authority”)—“Professor Bloggins says pigs can fly.”
    • Argumentum ad consequentiam (“appeal to the consequences”)—“A world where pigs can fly would be an awesome place, so I'll believe that they can.”
    • Argumentum ad baculum (“appeal to force” or, literally, “appeal to the stick”)—“If you don’t agree that pigs can fly, I’ll punch you in the nose.”
    • Argumentum ad nauseam —Finally, ridiculous though my case may be, if I continue pressing it long enough, everyone else will get sick of arguing with me.

Setting aside the arguments of questionable relevance we’ve made in favor of flying pigs, the varied types of faulty reasoning also constitute fallacies of their own.

Offering premises that give no support to one’s conclusion is called a non sequitur (“It does not follow”). “His dissertation was excellent, since he served doughnuts at the seminar where he presented it.” Doughnuts are good in and of themselves, but they have nothing to do with whether the dissertation was good or poor.

Affirming the consequent is the fallacy that arises from the mistaken belief that “if X, then Y” also means “if Y, then X.” “When it snows, it’s cold outside. It’s cold outside, therefore it must be snowing.”

Finally, though begging the question is often taken to mean “"raising the question” nowadays, it refers to a circular argument in which the conclusion appears as one of the premises. In other words, one assumes at the outset what one is supposed to be proving. “Bob is always right.” Why? “Because Bob says so, and he’s

User Comments

shrinivas Kulkani's picture
shrinivas Kulkani

You missed "symbolic Logic" - also how does all of it ties to software testing

Shrinivas Kulkarni


October 10, 2011 - 4:05am
Harry Theus's picture
Harry Theus

This article was both informative and entertaining. I enjoyed reading it.

September 22, 2011 - 11:36am

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