Hilgard begins his essay by denying the unity of consciousness. Partial consciousness involves selecting some features to attend to, while withdrawing attention from others. Hilgard's work on this problem has involved the study of hypnosis for almost forty years. In the 1960's, he found the concepts of dissociation and partial dissociation useful to explain divided attention and the effects of hypnotic suggestion on attention. In the 1970's, however, an unplanned demonstration involving a particularly hypnotizable subject led him to posit "the hidden observer" and to formulate the neodissociation hierarchical model.