The Macro-objectification Problem and Conscious Perceptions GianCarlo Ghirardi ∗ Emeritus, Department of Physics, the University of Trieste, the Abdus Salam International Centre for Theoretical Physics, Trieste, and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy. Abstract We reconsider the problem of the compatibility of our definite perceptions with the linear nature of quantum theory. We review some proposed solutions to the puzzling situation implied by the possible occurrence of superpositions of different perceptions and we argue that almost all are not satisfactory. We then discuss the way out which makes explicit reference to consciousness and we underline its pros and cons. In the second part of the paper we reconsider this problem in the light of the recently proposed collapse models, which overcome the difficulties of the standard theory by adding nonlinear and stochastic terms to the evolution equation and, on the basis of a unique dynamical principle, account both for the wavy behaviour of microsystems as well as for definite macroscopic events. By taking into account that different microscopic situations can trigger different displacements of an enormous number of particles in our brains which, in turn, lead to different and definite perceptions, we make plausible that such models do not assign a peculiar role to the conscious observer. Simply, the characteristic amplification mechanism leading to the collapse implies the suppression of all but one of the nervous stimuli corresponding to different perceptions. Thus, collapse models, at the nonrelativistic level, qualify themselves as theories which can consistently account for all natural processes, among them our definite perceptions. Part I 1 A sketch of the quantum description of physical processes Since the galilean revolution, the natural language of any scientific theory has been math- ematics. In particular, different physical situations characterizing an individual physical system are usually described by appropriate mathematical entities which uniquely spec- ify the “state” of the system under consideration. A paradigmatic example is given by * e-mail: ghirardi@ts.infn.it 1