Animal and human data suggest that neuroleptic drugs act by decreasing the special significance of some stimulations, especially those of importance for the subject. The two following hypotheses are therefore considered as a basis for the work presented in this paper: 1) the target of neuroleptic drugs is a neuro-psychological mechanism whose function is to focalize the subject on important stimulations and to involve the subject in responding; 2) schizophrenia vulnerability could imply an excessive and abnormal propensity to be over-involved, especially in existentially loaded problems. A careful clinical study of the mode of decrease, and sometimes of disappearance, of schizophrenic delusions under the influence of neuroleptic treatment confirms the hypotheses. Results are of interest for clinical treatment and for research.