The authors report on two experiments designed to test an important feature of the primary process: unconscious categorisation by attributes rather than by relations. These experiments were designed to provide support, independently of the clinical situation, for the presupposition of a psychological unconscious and for the presupposition that unconscious mentation is organised along primary-process lines. Their results were encouraging. They found that (1) unconscious similarity judgements could be made; and (2) these judgements were based on attributes (a primary-process mechanism) rather than relationships (a secondary-process mechanism). This independent evidence, obtained in controlled experimental studies supporting two fundamental psychoanalytic presuppositions, should be welcome news to psychoanalysts, given the continuing criticism from many quarters that basic psychoanalytic ideas lack independent validation. This paper begins with an overview of the primary processes with a special focus on the role of categorisation by attribute, the particular aspect of primary process explored in this study. Next a brief history of previous empirical investigations of primary process is given, following which the current experiments are presented.