54 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



The greater portion of our mental activity consists in 

 the formation of a continual flow or succession of concep- 

 tions of simple and complex ideas, excited in our minds by 

 phenomena occurring within and without us, and modified 

 more or less in strength at varied intervals by our acts 

 of volition. We cannot, however, by an act of the will, 

 directly prevent the flow of ideas. Ideas arrive and depart 

 at a certain average rate, and within certain limits of speed 

 of succession ; and if it were not so, mental action would 

 be either less po werful or less perfect. A sufficient number 

 of ideas, and proper time for digesting them, are both ne- 

 cessary to mental health ; an insufficient number or variety 

 would leave us in comparative ignorance, and a multiplicity 

 of them would confuse our minds. We can neither confine 

 our attention to a perfectly unchanging idea for a long 

 time together, nor clearly perceive those that remain before 

 us too short a time. As rapid irregular motion confuses 

 the vision, so a rapid succession of ideas confuses the mind. 

 Reflection is a tranquil flow of volitional thoughts. The 

 flow of ideas or thought is considered to be dependent 

 upon the travelling about of currents of nerve-power from 

 cell to cell of the cortical grey matter of the cerebrum. 

 When one idea appears, the preceding one disappears ; ideas 

 also disappear when the nervous current discharges out- 

 wards to produce a muscular movement, a flow of tears, 

 &c., or other external action. As by directing our atten- 

 tion to powerful or exciting ideas, we are able to strengthen 

 the current of thought, so can we, by directing it only to 

 feeble or non-exciting ones, promote the occurrence of 

 sleep and unconsciousness. 



The degree of rapidity of thought or flow of ideas 

 differs greatly in different persons, and in the same person 

 at different times ; and appears to be dependent upon the 

 speed of nervous conduction and cerebral impression. This 



