J.—PSYCHOLOGY. 225 
accidental consequence of conditions subsequent to birth. In the 
former case the backwardness, being inherent, is therefore incurable; 
in the latter, there remains at least a hope that, by amending the un- 
favourable circumstances, the backwardness may be partly remedied 
or even wholly removed. 
a, General Intelligence. 
We have now narrowed our scope for the moment to qualities that 
are intellectual and at the same time inborn; at this point we may 
apply our third and last distinction. Inborn intellectual abilities are 
divisible, first, into a single central capacity, pervading all that we say 
or think or do; and, secondly, into a series of specific abilities, each 
entering only into processes of a more or less limited kind. 
For the existence of general inborn intellectuai ability (known briefly 
as ‘ intelligence ’) the statistical evidence is now pretty decisive. Even 
the critics of this so-called central factor no longer deny that, at least as 
a matter of mathematical interpretation, the empirical data may be 
formulated in these terms; and that this formulation, whatever its 
ultimate psychological explanation, is of the greatest value in practice, 
and, as a working hypothesis, works very well. 
If further proof were demanded, the indubitable success of intelli- 
gence-testing has supplied a widespread verification, sufficiently 
business-like to convince the plain man. Indeed, over the whole realm 
of mental science the outstanding feat of recent years has been the 
application and the multiplication of innumerable tests for measuring 
general ability. As everybody knows, during the War the intelligence 
of nearly two million recruits was tested by these means for the Army 
of the United States. And this spectacular achievement has probably 
bestowed on the practical applications of psychological methods a 
stronger impetus than any other single piece of work. 
Since intelligence, as we have defined it,** is an inborn quantity, 
the amount possessed by a given individual should, in theory, remain 
constant through all the years of his life. It should thus be possible 
to predict, from quite an early age, what will be the probable intel- 
lectual level of a child when he is grown up. Within reasonable limits 
such forecasts can, in fact, be made. Numerous investigations have 
shown that what is called the ‘ mental ratio ’"—the proportion, that is, 
between a child’s mental age and his chronological age—tends to keep 
tolerably uniform throughout the years of growth. Hence it is safe 
‘to prophesy that a child (for example), aged five by the calendar, with 
a mental age of two (and a mental ratio, therefore, of 2=4() per cent.), 
will probably attain a mental age of four at the age of ten, and a mental 
18 The reader will understand that intelligence in this sense is not to be 
conceived as a concrete organ, entity, or power, but a purely abstract poten- 
tiality—like electrical energy or heat as conceived by the physicist—an entirely 
hypothetical quantity, postulated and defined, like most other scientific concepts, 
for the convenience of separate measurement. It is to be distinguished from 
manifested intelligence (the materialisation, as it were, of that abstract poten- 
tiality), which develops during childhood and decays with loss.of health or 
advance of age, and is measurable in terms of mental years or of some more 
concrete unit, 
R2 
