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and, through its medium, on our consciousness. To their 

 incessant operation, indeed, we owe the continuance of our 

 existence, and all the enjoyments and pains with which it 

 is replete. The external form of the different parts of our 

 body, and all our organs of sense, relate to the properties 

 of the objects around us. The structure of our internal 

 organs, and every faculty and instinct we possess, have an 

 intimate relation to the objects of the external world. The 

 elaborate mechanism of the eye would have been useless 

 without light, the ear without the vibrations of the air, 

 and the delicate structure of the organ of smell without the 

 odorous emanations of external bodies. We have conti- 

 nually recourse to external objects to supply the necessities 

 of the body, to repair its losses, and to remedy the injuries 

 it sustains. We ascribe to impressions received from them 

 all the sensations, ideas, or states of the consciousness, 

 which form the first elements of our knowledge, and the 

 materials of all our most complex mental operations. They 

 administer to all 'the wants of the body, to the pleasures of 

 sense, and to the most refined intellectual enjoyments ; so 

 that, in our present state of being, the objects of the mate- 

 rial world are intimately connected even with our intellec- 

 tual existence, with all our instincts and passions, and with 

 every internal feeling of pleasure or of pain. 



These objects, however, are very differently related to 

 us by their natural properties ; some are indifferent to us 

 with regard to pleasure or pain, or to the wants of our 

 body ; others are productive of agreeable sensations, or are 

 essential to our existence ; and others, by their noxious 

 qualities, are injurious or destructive to our nature, and 

 require to be carefully distinguished and avoided. The 

 study, therefore, of the distinguishing properties, and the 

 various relations of natural bodies, forms a most suitable 

 and important occupation of the human mind. 



By means of our senses we are admitted to an acquaint- 

 ance with the external world, and are enabled to dis- 

 tinguish, by their sensible qualities, the objects which we 



