Boolean Logic is NOT Logical

“Boolean logic refers to the logical relationship among search terms, and is named for the British-born Irish mathematician George Boole.”

“Boolean logic allows you to combine words and phrases into search statements to retrieve documents from searchable databases.” (Cohen) 

To use Boolean logic, you have to stop thinking in the English language or in the language of math.  When we say the word AND or when we use the plus sign (+), we make more - we expand our  universe. Dogs AND Cats (Dogs + Cats) contain more animals than just Dogs or just Cats, right?

But in Boolean logic, AND actually limits your results - you end up with less: Dogs AND Cats (Dogs + Cats)  gives you only the intersection of the two (as shown in the Venn diagram).

And in English, when we use the word OR, we are limiting our universe so that we have less. Either Dogs OR Cats (but not both of them together). Well,  guess what? the Boolean OR expands your results - that's right...you get more as a result:

The Boolean OR is useful when you want to search for synonymous terms at the same time (example: personal learning network OR PLN).

WHY is this important, you might ask? Because when you use any Search Engine, you need to think like a computer - not like a human! Search Engines use Boolean logic - not Human logic! Have you ever searched  for  some  words as a string of terms and NOT received the  results you expected? That's because the default logical operator might not match your logic and because not all Search Engines use the same default operator. [If you do not specify your operator between terms, the search is run using that engine's default operator.] Google, Bing, and Yahoo! all use AND as the default when searching for more than one term.

Another logical concept is NOT or the minus sign (-); this one is a limiter in both English and Boolean. This one must be applied with care because the results you are eliminating by the NOT operation might indeed be relevant (even valuable).

Reference:

Cohen, Laura B. "Boolean Searching on the Internet." Internet Tutorials. N.p., n.d. Web. 19 Oct. 2012. <http://www.internettutorials.net/boolean.asp>.