Class III is Bentham’s idioscopic; that is, the special sciences, depending upon special observation, which travel or other exploration, or some assistance to the senses, either instrumental or given by training, together with unusual diligence, has put within the power of its students. This class manifestly divides itself into two subclasses, the physical and the psychical sciences; or, as I will call them, physiognosy and psychognosy. Under the former is to be included physics, chemistry, biology, astronomy, geognosy, and whatever may be like these sciences; under the latter, psychology, linguistics, ethnology, sociology, history, etc. Physiognosy sets forth the workings of efficient causation, psychognosy of final causation. But the two things call for different eyes.
Idioscopy has two wings: (α) the Physical Sciences; and (β) the Psychical, or Human Sciences.
Psychical science borrows principles continually from the physical sciences; the latter very little from the former.
Psychognosy is that branch of idioscopy which observes the evidences of beings like ourselves and reasons upon them.