INTERMIXTURE OF EFFECTS. 523 



words, to press downwards with a force only equal to 

 the difference of the two forces. 



These facts are correctly indicated by the expres- 

 sion tendency. All laws of causation, in consequence 

 of their liability to be counteracted, require to be 

 stated in words affirmative of tendencies only, and not 

 of actual results. In those sciences of causation 

 which have an accurate nomenclature, there are spe- 

 cial words which signify a tendency to the particular 

 effect with which the science is conversant ; thus 

 pressure, in mechanics, is synonymous with tendency 

 to motion, and forces are not reasoned upon as causing 

 actual motion, but as exerting pressure. A similar 

 improvement in terminology would be very salutary 

 in many other branches of science. 



The habit of neglecting this necessary element in 

 the precise expression of the laws of nature, has given 

 birth to the popular prejudice that all general truths 

 have exceptions ; and much unmerited distrust has 

 thence accrued to the conclusions of philosophy, when 

 they have been submitted to the judgment of persons 

 who were not philosophers. The rough generaliza- 

 tions suggested by common observation usually have 

 exceptions ; but the principles of science, or in other 

 words, the laws of causation, have not. "What is 

 thought to be an exception to a principle," (to quote 

 words used on a different occasion,) " is always some 

 other and distinct principle cutting into the former; 

 some other force which impinges against the first force, 

 and deflects it from its direction. There are not a 

 law and an exception to that law, the law acting in 

 ninety-nine cases and the exception in one. There 

 are two laws, each possibly acting in the whole hun- 

 dred cases, and bringing about a common effect by 

 their conjunct operation. If the force which, being 



