As our senses are at present constituted, they are too imperfect, to enable us to distinguish all the elements, that co-exist in bodies, and of elements, which are themselves unknown to us, the minute changes which take place in them, must of course be unknown. We are hence, from our incapacity of discovering these elements by our imperfect senses, and imperfect analysis, incapable of distinguishing the whole series of external changes that occur in them,--the whole progressive series of antecedents and consequents in a phenomenon that appears to our senses simple; and, since it is only between immediate antecedents and consequents, that we suppose any permanent and invariable relation, we are therefore constantly on the watch, to detect, in the more obvious changes that appear to us in nature, some of those minuter elementary changes, which we suspect to intervene. These minute invisible changes, when actually intervening, are truly what connect the obvious antecedents with the obvious consequents; and the innumerable discoveries, which we are constantly making of these, lead us habitually to suppose, that, amid all the visible changes perceived by us, there is something latent which links them together. He who for the first time listens to the delightful sounds of a violin, if he be ignorant of the theory of sound, will very naturally suppose that the touch of the strings by the bow is the cause of the melody which he hears. He learns, however, that this primary impulse would be of little effect, were it not for the vibrations excited by it in the violin itself; and another discovery, still more important, shews him that the vibration of the instrument would be of no effect, if it were not for the elastic medium, interposed, between his ear and it. It is no longer to the violin, therefore, that he looks, as the direct cause of the sensation of sound, but to the vibrating air; nor will even this be long considered by him as the _cause_, if he turns his attention to the structure of the organ of hearing. He will then trace effect after effect, through a long series of complex and very wonderful parts, till he arrive at the auditory nerve, and the whole mass of the brain,--in some unknown state of which he is at length forced to rest, as the cause or immediate antecedent, of that affection of the mind, which constitutes the particular sensation. To inquire into the latent causes of events is thus to endeavour to observe changes which we suppose to be actually taking place before us unobserved, very nearly in the same manner, as to inquire into the composition of a substance is to strive to discover the bodies that are constantly before us, without our being able to distinguish them.