J.—PSYCHOLOGY 187 
(unless they be twins) ; families move from easy to difficult circumstances 
and vice versa ; parents become more experienced, or more indulgent, in 
the management of their children ; school-fellows vary ; and the children 
themselves vary in their relationships to one another and to the rest of the 
world. The conditions of the experimental chemical laboratory cannot 
be exactly reproduced in the study of human and social phenomena ; we 
have to be content with approximations to these conditions. 
It is necessary to stress these considerations of method, for psychologists 
have hitherto been more concerned to distinguish and measure different 
kinds of ability which seem to be dependent on native capacity than to 
prove their innate basis. An example may make this clear. Itisacommon 
belief that people differ in respect of mechanical ability, that some have 
little difficulty in understanding the working of a motor car, a dynamo, 
a clock or other piece of mechanism, and that others find these things 
unintelligible ; it is also commonly believed that these differences are 
due to differences in natural capacity. Now, the first thing that must 
be done is to find whether there is actually a positive correlation between 
ability to solve one kind of mechanical problem and ability to solve 
other kinds, for until such a correlation has been established, it is 
futile to talk about mechanical ability. This is the kind of problem on 
which much effort has been spent, especially in this country : but after 
a correlation has been established, it is still necessary to find to what extent 
this ability is the expression of a specific inborn capacity. This more 
difficult problem is usually attacked by using test situations so novel that 
there is little probability of one examinee having any advantage over 
another through familiarity with the situation, or by using problems such 
as occur so often that it can be presumed that inability to solve them is due 
ultimately to innate incapacity. In practice, the difficulty, once it has 
been recognised, is probably not so great as may appear, for the opportuni- 
ties of and the need for exercising most of one’s native capacities are in 
fact numerous ; a person who fails to pass a properly designed and pro- 
perly conducted test of colour blindness is almost certainly colour-blind. 
All kinds of capacities are being investigated with varying success, and 
it may be possible some day to evaluate mental characters with some 
approximation to the accuracy with which physical characters can be 
assessed. What is needed is more extensive and more co-operative work. 
Most progress has been made in the evaluation of intellect by the so-called 
intelligence tests, largely under the pressure of educational needs. 
Intelligence tests, as developed by Binet, were simply tests of educabi- 
lity, methods of picking out those children who are incapable of profiting 
from the education provided in the ordinary primary school. They have 
done more than this, for they have provided a method of distinguishing 
all degrees of general capacity. In principle they are just a refinement of 
a very common method of estimating native brightness. Binet put to 
children questions about topics which were likely to come within their 
everyday experience; he found what average children of different ages 
could do and was able to arrange his questions in a scale of increasing 
difficulty ; then he assumed that those who picked up the necessary 
information or acquired the necessary skill or showed the necessary 
