I don’t understand why you don’t agree with me.
A New York Times* online essay by Gary Gutting, a philosophy professor, contains one of the clearest explanations of the differences between inductive and deductive reasoning I have seen.
The answer lies in a crucial distinction between deductive and inductive reasoning. In a deduction (e.g., all humans are mortal; Socrates is human; therefore, Socrates is mortal), the truth of the premises logically requires the truth of the conclusion. If the premises are true, the conclusion must be true. Adding further premises that might seem relevant to the conclusion (e.g., Socrates is very young and there will be major medical advances before Socrates reaches old age) will make no difference to the conclusion.
In an inductive argument (e.g., Most humans do not live for 100 years; Socrates is human; therefore, Socrates will not live for 100 years), the premises only make the conclusion probable. As a result, adding further premises can alter the force of the argument. For example, if Socrates is 99 years old and in very good health, it is probable that he will live to be 100.
Even a strong argument from purely factual premises is open to refutation unless we are assured that it has taken account of all relevant facts. Realistically, of course, we can never be sure that we have taken account of all relevant facts, especially with an issue as complex as a national budget. But a good inductive argument requires getting as close as we can to this ideal. . . .
Ignoring relevant facts can give us false confidence in the strength of our positions in political debates. I put forward a barrage of indisputable facts that show, with a very high probability, that my view is correct. But you construct an equally impressive argument refuting my view. . . . Each of us may conclude that the other is irrational or ignorant. But we should beware of the sense of the inviolability of our own positions when what we really need is a serious effort to argue from all the relevant facts.
If that knocked your socks off, take a look at our next cool topic, Things to worry about. And if you want to peruse all of the previous sock-knocking blog entries, visit the Knocked My Socks Off archive (links to another blog site).
* Gutting G. Arguing from the facts. Opinionator: online commentary from The New York Times. July 6, 2011. http://opinionator.blogs.nytimes.com/2011/07/06/arguing-from-the-facts/?scp=1&sq=arguing%20from%20the%20facts&st=cse. Accessed January 7, 2013.