316 



INDtJCTION. 



arts have usually been founded, are 

 continually justified and confirmed on 

 the one hand, or corrected and im- 

 proved on the other, by the discovery 

 of the simpler scientific laws on which 

 the efficacy of those operations de- 

 pends. The effects of the rotation of 

 crops, of the various manures, and 

 other processes of improved agricul- 

 ture, have been for the first time re- 

 solved in our own day into known 

 laws of chemical and organic action 

 by Davy, Liebig, and others. The 

 processes of the medical art are even 

 now mostly empirical : their efficacy 

 is concluded, in each instance, from a 

 special and most precarious experi- 

 mental generalisation : but as science 

 advances in discovering the simple 

 laws of chemistry and physiology, 

 progress is made in ascertaining the 

 intermediate links in the series of 

 phenomena, and the more general laws 

 on which they depend ; and thus, 

 while the old processes are either ex- 

 ploded, or their efficacy, in so far 

 as real, explained, better processes, 

 founded on the knowledge of proxi- 

 mate causes, are continually suggested 

 and brought into use.* Many even 

 f the truths of geometry were gene- 

 ralisations from experience before 

 they were deduced from first prin- 

 ciples. The quadrature of the cycloid 

 is said to have been first effected by 

 measurement, or rather by weighing 

 a cycloidal card, and comparing its 

 weight with that of a piece of similar 

 card of known dimensions. 



§ 6. To the foregoing examples from 

 physical science let us add another 

 from mental. The following is one 

 of the simple laws of mind : Ideas of 

 a pleasurable or painful character 



* It was an old generalisation in surgery 

 that tight bandaging had a tendency to pre- 

 vent or dissipate local inflammation. This 

 sequence being, in the progress of physio- 

 logical knowledge, resolved into more gene- 

 ral laws, led to the important surgical in- 

 vention made by Dr. Arnott, the treatment 

 of local inflammation and tumours by 

 means of an equable pressure, produced by 

 a bladder partially filled with air. The 

 pressure, by keeping back the blood from 



form associations tnore easily and 

 strongly than other ideas, that is, 

 they become associated after fewer 

 repetitions, and the association is 

 more durable. This is an experi- 

 mental law, grounded on the Method 

 of Difference. By deduction from 

 this law, many of the more special laws 

 which experience shows to exist among 

 particular mental phenomena may be 

 demonstrated and explained : — the 

 ease and rapidity, for instance, with 

 which thoughts connected with our 

 passions, or our more cherished in- 

 terests are excited, and the firm hold 

 which the facts relating to them have 

 on our memory ; the vivid recollec- 

 tion we retain of minute circumstances 

 which accompanied any object or event 

 that deeply interested us, and of the 

 times and places in which we have 

 been very happy or very miserable ; 

 the horror with which we view the 

 accidental instrument of any occur- 

 rence which shocked us, or the locality 

 where it took place, and the pleasure 

 we derive from any memorial of past 

 enjoyment ; all these effects being 

 proportional to the sensibility of the 

 individual mind, and to the consequent 

 intensity of the pain or pleasure from 

 which the association originated. It 

 has been suggested by the able writer 

 of a biographical sketch of Dr. Priest- 

 ley in a monthly periodical,* that the 

 same elementary law of our mental 

 constitution, suitably followed out, 

 would explain a variety of mental 

 phenomena previously inexplicable, 

 and in particular some of the funda- 

 mental diversities of human character 

 and genius. Associations being of 

 two sorts, either between synchronous, 

 or between successive impressions ; 

 and the influence of the law which 



the part, prevents the inflammation, or the 

 tumour, from being nourished : in the case 

 of inflammation, it removes the stimulus, 

 whicii the organ is unfit to receive ; in the 

 case of tumours, by keeping back the 

 nutritive fluid, it causes the absorption of 

 matter to exceed the supply, and the 

 diseased mass is gradually absorbed and 

 disappears. 



* Since acknowledged and reprinted in 

 Mr. Martineau's MUceUanies. 



