EXAMPLES OF THE EXPLANATION OF LAWS. 343 



founded, are continually justified and confirmed on the one hand, or cor- 

 rected and improved on the other, by the discovery of the simpler scientific 

 laws on which the efficacy of those operations depends. The effects of the 

 rotation of crops, of the various manures, and other processes of improved 

 agriculture, have been for the first time resolved 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 generalization : but as science advances in discovering the simple 

 laws of chemistry and physiology, progress is made in ascertaining the in- 

 termediate links in the series of phenomena, and the more general laws on 

 which they depend ; and thus, while the old processes are either exploded, 

 or their efficacy, in so far as real, explained, better processes, founded on 

 the knowledge of proximate causes, are continually suggested and brought 

 into use.* Many even of the truths of geometry were generalizations from 

 experience before they were deduced from first principles. The quadra- 

 ture 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 form associations more easily and strong- 

 ly than other ideas, that is, they become associated after fewer repetitions, 

 and the association is more durable. This is an experimental law, ground- 

 ed on the Method of Difference. By deduction from this law, many of the 

 more special laws which experience shows to exist among particular men- 

 tal phenomena may be demonstrated and explained : the ease and rapid- 

 ity, for instance, with which thoughts connected with our passions or our 

 more cherished interests are excited, and the firm hol4 which the facts re- 

 lating to them have on our memory; the vivid recollection 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 occurrence which shocked us, or the locality where it 

 took place and the pleasure we derive from any memorial of past enjoy- 

 ment ; all these effects being proportional to the sensibility of the individ- 

 ual 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. Priestley 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 fundamental diversities of human character and 

 genius. Associations being of two sorts, either between synchronous, or 



* It was an old generalization in surgery, that tight bandaging had a tendency to prevent 

 or dissipate local inflammation. This sequence, being, in the progress of physiological knowl- 

 edge, resolved into more general laws, led to the important surgical invention made by Dr. 

 Arnott, the treatment of local inflammation and tumors by means of an equable pressure, 

 produced by a bladder partially filled with air. The pressure, by keeping back the blood 

 from the part, prevents the inflammation, or the tumor, from being nourished : in the case of 

 inflammation, it removes the stimulus, which the organ is unfit to receive ; in the case of tu- 

 mors, by keeping h-.u-k the nutritive fluid, it causes the absorption of matter to exceed the 

 supply, and the diseased mass is gradually absorbed and disappears. 



t Since acknowledged and reprinted in Mr. Martineau's Miscellanies. 



