Patrick Stokes at ABC:
Conspiracy theories weren't invented by the internet. They go back at least as far as the elite reaction to the French Revolution, with a grand Illuminati-Masonic conspiracy theory taking hold on both sides of the Atlantic before the start of the 19th century. Anti-Semitic conspiracy theories had tragic consequences during the last century, while today the Obama administration has had to contend with everything from demands for the president's birth certificate to state governments fuelling rumours of impending martial law. The consequences of conspiracy theories are, as they have always been, concrete and significant.
Most of us use the term 'conspiracy theory' to refer to beliefs we consider outlandish, paranoid, and almost certainly false. Yet strictly speaking this is unfair: on the simplest definition, a conspiracy theory is simply any explanation of observed events that posits two or more actors working in secret. Philosophers who have considered conspiracy theories as a class of explanation insist that there's nothing intrinsically irrational about conspiracy theory so defined. In fact, if we didn't accept the idea of a group of actors plotting in secret, we'd be unable to explain a host of historical events, from the assassination of Julius Caesar to Watergate. Conspiracies happen.