Scientific introspection is necessary because of a known unknown in the world of experimentation called experimenter bias, the name given to all the ways in which simply being human can affect how scientists take data, analyze data, and interpret data. “Experimenter bias” is a known—i.e. it is well-known to exist—but it’s also unknown, because the psychology of the human scientist can’t be easily quantified. Researchers can’t add error bars indicating the strength of a preconceived notion or the weight of an unconscious desire, yet subtle and not-so-subtle influences definitely exist, such as the result of a previous experiment or the conclusion of a popular theory, and these influences can nudge a researcher into accepting a result prematurely or discounting results that don’t conform to expectation.