NECESSITY OF EDUCATING THE MEMORY. 73 



of contiguity, such as that supplied by a general principle. 

 To discipline the memory thoroughly, and especially to 

 acquire a ready and accurate use of our knowledge, the 

 ideas should be recalled from time to time by practice in 

 speaking or writing, for Lord Bacon said, ' Eeading makes 

 a full man ; writing, an accurate man ; and speaking, a 

 ready man.' Ideas revived by the memory, especially 

 during the first half of one's life, may be made, by means 

 of study and repetition, even more vivid and enduring than 

 the original impressions of them. Our oldest thoughts 

 are often the most enduring, partly because the sensorium 

 and cerebrum of young persons are usually more receptive 

 of impressions, and partly because the later formed parts of 

 our physical structure are those which most early dege- 

 nerate and decay. Memory has also numberless diseases 

 and affections which it is unnecessary for me to describe. 



CHAPTER VI. 



ON SCIENTIFIC TERMS. 



EVERY clear scientific idea is the result of a definite act of 

 mental power, and a precise portion of existing knowledge 

 or belief, and its limits are indicated by its essential 

 marks and characteristics, and not by its coincident or 

 accidental associations. Each object and idea also is dis- 

 tinguished from all other objects and ideas by those 

 characteristic marks only. To distinguish a metal, there- 

 fore, we need only to know the characteristic signs of a 

 metal ; and to recognise copper, we require to know only 

 the distinguishing marks of that substance. When we 

 distingush, we show a difference. ' All arts acknowledge 

 that then only we know certainly, when we can define ; 



