"The key to a scientific inquiry into the nature of the animal soul is evidently the soul of man. For we have no immediate insight into the psychic acts of the animal; we can only infer their existence and nature from the exterior actions which our senses perceive. We must compare these manifestations of the activity of the animal soul with the manifestations of our own psychic life, the interior causes of which are known to us from our inner consciousness. Consequently scientific psychology applies the same key as pseudo-psychology, but it follows critical method."