Gould argues that the primary assumption underlying biological determinism is that, “worth can be assigned to individuals and groups by "measuring intelligence as a single quantity"”. Biological determinism is analyzed in discussions of craniometry and psychological testing, the two principal methods used to measure intelligence as a single quantity. According to Gould, these methods possess two deep fallacies. The first fallacy is reification, which is “our tendency to convert abstract concepts into entities”. Examples of reification include the intelligence quotient (IQ) and the general intelligence factor ("g" factor), which have been the cornerstones of much research into human intelligence. The second fallacy is that of “ranking”, which is the “propensity for ordering complex variation as a gradual ascending scale”.