This question, the author says, cannot be treated from a theoretic or metaphysical point of view; it ought to be dealt with, on the contrary, by the positive method of observation and experience. One has been taught that psychological life is bound up with the presence of a nervous system. As we descend the zoological scale the nervous system gradually simplifies its structure and finally disappears. No one has demonstrated with certainty the presence of nervous substance in the protozoa. Are we, therefore, to conclude that there is no psychological life in these simple animals? The author considers that the continuity, which in zoology binds the most simple phenomena to the most complex, leads us to think that even among the infusoria there may be rudimentary psychological phenomena.