A brief historical review of the concept of delusions illustrates the complexity of the issues involved, and the difficulty in obtaining consensus on the concept. Five characteristics of delusions are cited. Of these, the author selected the qualities of fixity and intensity of belief to examine in detail by use of a formal psychological protocol. Both intensity and fixity of belief were observed to fluctuate in subjects who were considered clearly delusional from a clinical point of view, suggesting that delusions should be considered phenomena at one end of a spectrum, rather than as discrete entities. The protocols allowed formal measurement of two aspects of delusions, thus making them accessible to systematic investigation.