ABSTRACT
The first, and not the least, of the difficulties presented by the study of the parapsychic phenomena is that these phenomena are not produced in an ordinary way, but allow themselves to be perceived only rarely and in. exceptional and abnormal circumstances. The truth is that, in order to study them, it usually is necessary for us to provoke them ourselves, artificially. But here experimentation encounters a new difficulty. The same processes do not succeed with all subjects, nor in all circumstances.
