"A poor relation is the most irrelevant thing in nature, a piece of impertinent correspondence, an odious approximation, a haunting conscience, a preposterous shadow, lengthening in the noon-tide of our prosperity. He is known by his knock."
Relevance is the connection between topics that makes one useful for dealing with the other. Relevance is studied in many different fields, including cognitive science, logic, and library and information science. Epistemology studies it in general, and different theories of knowledge have different implications for what is considered relevant.
"I feel that any form of so called psychotherapy is strongly contraindicated for addicts. The question "Why did you start using narcotics in the first place?" should never be asked. It is quite as irrelevant to treatment as it would be to ask a malarial patient why he went to a malarial area."
"A few pen-and-ink sketches of scenery and character may not be irrelevant; as serving to mark, palpable contrasts between the past and present, country and town, nature and art, illiterate and literary society - man almost in a state of semi-barbarism, and man enlightened by art, science, and refinement."
"A theory has only the alternative of being right or wrong. A model has a third possibility: it may be right, but irrelevant."
"Its irrelevant who or what directed a movie, the important thing is that you either respond to it or you dont."