Raymond Cattell's trait theory began by narrowing down a vast list of personality descriptors to 171. A central idea of his approach is that personality is not defined by the presence or absence of traits, but by the degree to which each trait is expressed on a continuum. He proposed that all individuals possess the same fundamental traits, with personal uniqueness arising from their different levels of expression. Through factor analysis, he identified 16 core personality dimensions and developed the 16PF assessment to measure them, viewing personality as a means to predict behavior.