EDUCATION AS A SCIENCE. 515 



cises ; we may indulge emotions, and carry out pursuits, and yet not 

 be in a state for storing the memory, or amassing knowledge. Even 

 the incidents that we take part in sometimes fail to be remembered 

 beyond a very short time. 



What, then, is there so very remarkable and unique in the physical 

 support of the plastic property of the brain ? What are the moments 

 when it is at the plenitude of its efficiency ? What are the things 

 that especially nourish and conserve it ? 



Although there is still wanting a careful study of this whole sub- 

 ject, the patent facts appear to justify us in asserting that the plastic 

 or retentive function is the very highest energy of the brain, the con- 

 summation of nervous activity. To drive home a new experience, to 

 make an impression self-sustaining and recoverable, uses up (we are 

 to suppose) more brain-force than any other kind of mental exercise. 

 The moments of susceptibility to the storing up of knowledge, the en- 

 graving of habits and acquisitions, are thus the moments of the maxi- 

 mum of unexpended force. The circumstances need to be such as to 

 prepare the way for the highest manifestation of cerebral energy ; in- 

 cluding the perfect freshness of the system, and the absence of every- 

 thing that would speedily impair it. 



To illustrate this position, I may refer to the kind of mental work 

 that appears to be second in its demand on the energy of the brain. 

 The exercise of mental constructiveness the solving of new prob- 

 lems, the applying of rules to new cases, the intellectual labor of the 

 more arduous professions, as the law, where a certain amount of nov- 

 elty attends every case that occurs demands no little mental strain, 

 and is easy according to the brain-vigor of the moment. Still, these 

 are exercises that can be performed with lower degrees of power ; we 

 are capable of such professional work in moments when our memory 

 would not take in new and lasting impressions. In old age, when we 

 cease to be educable in any fresh endowment, we can still perform 

 these constructive exercises ; we can grapple with new questions, 

 invent new arguments and illustrations, decide what should be done 

 in original emergencies. 



The constructive energy has all degrees, from the highest flights 

 of invention and imagination down to the point where construction 

 shades off into literal repetition of what has formerly been done. The 

 preacher in composing a fresh discourse puts forth more or less of 

 constructiveness; in repeating prayers and formularies, in reading 

 from book, there is only reminiscence. This is the third and least 

 exigent form of mental energy; it is possible in the very lowest states 

 of cerebral vigor. When acquisition is fruitless, construction is pos- 

 sible ; when a slight departure from the old routine passes the might 

 of the intelligence, literal reminiscence may operate. 



Another mode of mental energy that we are equal to, when the 

 freshness of our susceptibility to new growths has gone off, is search- 



