THK THIdKY 01- !SENSATIOy. 17? 



compared the mind to a daik cave in which a man was so placed that 

 he could look only towards its inner boundary. The light entered 

 from behind, between which and the man persons passed and re- 

 passed, their shadows beinu projected on the inner wall of the cavity. 

 It was these shadows, and not the objects tliemselves, which affected 

 the mind in producing sensation. 



The system of Aristotle and his followers, with Epicurus, was 

 briefly as follows : All things were supposed to be constituted 

 according to an original model or idea, which was termed its sub- 

 stantial fonn, and was supposed to account for all phenomena. From 

 the surfaces of all bodies were supposed to be continually passing off 

 shadowy films, detennined in their character by the substantial form. 

 These shadowy films were an exquisitely refined species of materi- 

 ality, and did not at all dimhiisli the weight or dimensions of the bodies 

 from which they were continually passing off. By an innate tendency 

 to motion they were impelled with inconceivable rapidity to the 

 organs of sense, and in consequence of the impression they pro- 

 duced on those organs were called sensible species. These species 

 or tiluXoc. of bodies, were received by the soul — a subtile eiherial 

 essence pervading every part of the frame, and were retained by it . 

 just as wax receives the impression of the seal, without abstracting 

 any part of its substance. These sensible species being remembered, 

 or rather the impressions of them on the mind being made the 

 objects of thought, were called phantasms, and being generalised 

 constituted the intelligible species. It was thus that the mind 

 obtained its sensation and thence its knowledge of the external world, 

 not from any direct agency of matter, but by the intervention of 

 these semi-spiritual phantasmata. 



All this insensible and unintelligible jargon of sensible and 

 intelligible species, was set aside in the seventeenth century by Des 

 Cartes, " that illustrious rebel," as Dr. Brown appropriately "terms 

 him, " who, in overthrowing the authority of Aristoile, seemed to 

 have acquired, as it were, by right of conquest, a sway in philosophy 

 as absolute, though not so lasting, as that of the Grecian despot." 

 Instead of pioceeding from external objects to what is mental he 

 made ** cogito" the foundation ot his system. Yet while he set out from 

 consciousness, he drew a strict line of demarcation between body 

 and spirit, maintaining that our knowledge of each must be derived 

 from an independent examination of each. He, therefore, abandoned 

 the ancient theory of sensation being produced by shadowy films and 

 phantasms, since for the existence of these "species" there was no 

 evidence either from consciousness or observation. According to the 

 Cartesian philosophy the soul has its seat in the pineal gland, this 

 being single in its construction ; and, therefore, suited for the residence 

 of what is simple and uncompounded. The impressions received by 

 the organs of sense are conveyed to the mind residing in the pineal 

 gland, by means of the animal spirits, a fluid which is supposed to 

 flow through the nerves which are supposed to be tubular. The eflects 

 thus produced on the brain are, according to Des Cartes, the occa- 

 sions, not the causes of sensation. This doctrine of occasional causes 



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