 The proposal to do without the projection postulate was also made by others, before van Fraassen published his work. In particular, Bohm’s hidden-variables theory (itself preceded by de Broglie’s proposals from the 1920s) and Everett’s (1957) relative-state scheme (elaborated by De Witt (1970) into the many-worlds interpretation) followed this path. According to Bohm’s theory, physical systems are built up from particles that always possess definite positions, while the wave function specifies the probabilities associated with the various possible values these positions may have. There are no collapses in Bohm’s theory: the wave function evolves unitarily at all times. Therefore, the theory qualifies as a modal interpretation of quantum mechanics: the state determines a range of possible particle configurations and specifies the probabilities for these possibilities to be actually realized.