In philosophy, a main understanding of what it means to be "rational" is: to be logical. And "logic" has a specific content. Conversely, "irrationality" means: "illogical," again with a very specific content.
One way reasoning goes astray is in using informal logical fallacies. These are sneaky, obscure little beasts that sound logical but are not. Informal logical fallacies add nothing to reasoning, are deceivers, and are, of course, false. In my MCCC Logic classes we just finished an introduction to symbolic logic. Now we're going to spend time teaching these irrational things.
The website Fallacy Files is a good place to enter into the sad world of informal logical fallacies.
Here, for example, is Michele Bachman using the "slippery slope fallacy."
Question: This is for all candidates. What's your position on replacing the federal income tax with a federal sales tax?
"As a reason to oppose Cain's proposal, Bachmann invokes a slippery slope from a 9% national sales tax to a 90% one. But just how slippery is that slope? To support the slipperiness of the slope, Bachmann gives the example of the income tax going from 7% to 70%. Let's examine that example more closely.Anderson Cooper: I'll direct that to Congresswoman Bachmann. You've been very critical of Herman Cain's 9-9-9 plan, which calls for a 9 percent sales tax, a 9 percent income tax, and 9 percent corporate tax. In fact, you've said it would destroy the economy. Why?Michele Bachmann: Well, I am a former federal tax litigation attorney. And also, my husband and I are job-creators. One thing I know about Congress, being a member of Congress for five years, is that any time you give the Congress a brand-new tax, it doesn't go away. When we got the income tax in 1913, the top rate was 7 percent. By 1980, the top rate was 70 percent. If we give Congress a 9 percent sales tax, how long will it take a liberal president and a liberal Congress to run that up to maybe 90 percent? Who knows?
Let's assume that Bachmann's numbers are correct―after all, this is a logic check, not a fact check. Why did she choose to compare the top income tax rate when the tax was adopted to that in 1980, that is, 31 years ago? Why didn't she compare the original top rate to the current one? I don't know, but the current highest marginal rate is 35%, according to Wolfram Alpha. This undercuts Bachmann's argument since, in the past three decades, the top rate has slid backwards to half of what it was.
So, what's to stop Cain's 9% national sales tax from ballooning to a 90% one? Presumably, the same thing that has prevented the income tax from doing so. Moreover, what's to stop Congress from adopting a national sales tax now? What has stopped it before now? Presumably, the usual mechanisms of democratic politics." (From here.)
I especially like the following, which diagrams informal logical fallacies - a "Taxonomy of Logical Fallacies." On the website, click on each one to get a description. Very nice!