Simon-Blalock method for causal inference by means of partical correlations was applied to two problems: one in space perception and another in motion perception, Experiment I dealt with size-distance invariance relation, and Experiment II, with the velocity-length-time, interrelation. The method can discriminate direct and indirect causal relation between stimulus variables and perceptual variables. In indirect relations, partial correlations between stimulus variables and perceptual variables with some intervening variables kept constant will become zero. The results of Experiment I indicated that, in the most subjects, both perceived size and perceived distance were determined directly by stimulus variables (visual angle and convergence). There was no direct relation between them. The results of Experiment II indicated that perceived velocity and perceeved path-length were determined directly by stimulus variables (angular velocity and visual angle of path). Perceived time was determined not only by a stimulus variable (angular velocity) but else by a perceptual variable (perceived path-Length).