1 What is rationality? (Paper presented by Tim Harding at Mordi Skeptics meetup, 1 February 2011) What do we skeptics mean when we say that a belief is irrational? How do we define rationality and irrationality? Are there any objective tests of an irrational belief? Firstly, some definitions. Most dictionaries define rationality as the state or quality of being rational. So what does it mean to be rational? Once again, most dictionaries define rational as being consistent with or based on or using reason,[1] which is further defined as the mental ability to draw inferences or conclusions from assumptions or premises (the ‘if – then’ connection). The application of reason is known as reasoning; the main categories of which are deductive and inductive reasoning.[2] Reason is thought by rationalists to be more reliable in determining what is true; in contrast to reliance on other factors such as authority, tradition, instinct, intuition, emotion, mysticism, superstition, faith or arbitrary choice (e.g. flipping a coin). For example, we rationally determine the balance in our cheque book (between bank statements) by adding up the credits and subtracting the debits and bank fees. An irrational way of doing it would be to pick a number at random – not very reliable, and any correct answer would be a mere coincidence, rather than the product of reasoning. The ancient Greeks thought that rationality distinguishes humans from other animals. “Man is a rational animal” as Aristotle said.[3] However, this distinction is becoming blurred by recent research indicating that other primate species such as chimpanzees can show a limited use of reason and therefore a degree of rationality. The word rational can be used in several different contexts; for example rational behaviour (psychology), rational or optimal decision (economics); a rational process (science), and rational belief (philosophy). However, it is not the purpose of this paper to discuss all uses of rationality – only those relevant to our use i.e. skepticism. I would suggest that the context most relevant to skepticism (which could be described as a form of applied philosophy) is that of rational belief, because we skeptics often criticise the beliefs of paranormals, quacks, cults and pseudo-sciences on the grounds that they are irrational (which, of course, is the antonym of rational).[4] However, the scientific context of a ‘rational process’ is also relevant to skepticism; and I will say more about this later. In my view, the relevance of rational belief to skepticism is that we use it as a filter to determine what we should be skeptical about. We skeptics are not skeptical of everything. We believe what it is rational to believe, and we are skeptical of beliefs that are known to be or appear to be irrational. That is why I think it is important for skeptics to clarify and understand the nature of rational belief.