The term "irrational beliefs" was introduced by Albert Ellis (1913-2007) as part of his Rational-Emotive Therapy treatment approach. They describe unrealistic expectations we can have about ourselves or the world which leave us feeling worse than we need to because they will invariably be disappointed at some point. Common irrational beliefs include the idea that everyone will like us, we should never make a mistake or act unkindly and that danger and unpleasantness must be avoided at all costs.
Rational-emotive therapy was the forerunner of today's popular Cognitive Behavior Therapy. Both look at how we interpret what's happening to and around us, and how this affects the way we feel. They differ from other forms of therapy in that they examine what someone is consciously thinking rather than exploring pure emotions or unconscious mechanisms. In cognitive models, people suffer unnecessarily when the meaning they give to events is unrealistic or biased, which can create negative self-fulfilling prophecies. When you believe, for example, that it's a bad day then you will pay extra attention to what goes wrong and the day will seem even worse.
Irrational beliefs are often rigid, over-generalized or extreme. If you hold that good people never do anything unkind or thoughtless, a single careless deed can brand someone a villain forever. A single mistake leaves you feeling like a total failure, a minor setback makes the whole world seem awful.
As well as listing popular irrational beliefs, Ellis also outlined certain "thinking errors" which give rise to them. Thinking errors are faults or gaps in the logic we use to figure things out. When we generalize, for instance, we try to establish a fact using too few examples. All-or-nothing thinking imagines that there are only two extreme possibilities with no shades of gray in between.
Strictly speaking, the terms "rational" or "reasoned" don't so much refer to ideas themselves as the process you use to reach your conclusions. A rational thought is one you have figured out logically; there's an observable connection between "A" and "B", and your conclusion fits known principles and all of the facts in your possession. Irrational thoughts are those which arrive through means other than a step-by-step reasoning, such as hunches' and vibes'. According to cognitive models, an unreasoned thought is vulnerable to the influence of negative moods and unrealistic expectations.
To free people from the grip of irrational beliefs, both Rational Emotive Therapy and Cognitive Therapy re-introduce logical reasoning to "dispute" the overly negative thoughts, although the two treatments differ slightly in their methods. Rational emotive therapy teaches people how to check their thinking for irrational beliefs and thinking errors. Cognitive Therapy uses a style more like a courtroom or parliamentary debate, weighing up the evidence for and evidence against a particular thought.
Our day-to-day interactions are filled with assumptions. They have to be. It's not possible to know everything; we'll always need to make a few guesses. Our assumptions are based on certain beliefs we have about ourselves, other people and the way the world works. Some, like "putting your hand on a hot stove will burn you" are very helpful beliefs! Others don't describe the world as well as they could, and give unhelpful results. Irrational beliefs often seek to make the world a simpler and more certain place, but wiping out its natural complexity tends to make it an unhappier one.