Introduction. The paper presents a critical examination of Professor Birger Hjørland’s relevance equation: Something (A) is relevant to a task (T) if it increases the likelihood of accomplishing the goal (G), which is implied by T.
Method. Two theories of probability logic (the logical theory and the intersubjective theory) are briefly reviewed and then applied to Hjørland’s equation.
Analysis. Focusing on how these theories warrant the probability assumption makes it possible to detect deficiencies in Hjørland’s equation, based as it is on probability logic.
Results. Regardless of the kind of logic applied to warrant the probability assumption of Hjørland’s equation, the outcome of using it to determine the relevance of any A to any T is found to have quite bizarre consequences: Either nothing is relevant or everything is relevant.
Conclusion. Contrary to Hjørland’s claim that his relevance equation applies to anything (including documents, ideas, meanings, texts, theories, and things), it is found at best to have very limited generalisability.