VARIOUS USES OF FACTORS	57
Thus, the heterogeneity of the tests in the Binet scale, so far from
diminishing its value, actually increases it; and the popular
batteries of written group tests, which are put forward as better
substitutes, and in which each test claims high correlations with the
rest, are, as a rule, far too homogeneous : they owe their high
correlation quite as much to what, with a more varied assortment of
test-material, would prove to be ^c overlapping specifics' (ease in
understanding the printed word, facility of verbal expression, and
the like) as to the ^c general factor ^5 of intelligence.
In educational work the psychologist's predictions often cover a
long range. In certifying the mentally defective we are by statute
required to show that the defect has existed " from birth or from an
early age " ; by the age of 6 or 7 we endeavour to discover those
who during their school career will never be able to profit by an
ordinary elementary education; towards the age of 10 or n we
seek to select others whose subsequent educational progress will
justify the award of a scholarship to a secondary school. In all
these instances of testing, examining, or diagnosing, we are trying
to ascertain, not (as the detective does) who has committed a
momentary act, but who possesses a given amount of some lasting
capacity. Since these educational predictions cover most forms of
intellectual work, the capacity we desire to estimate must be a
^c general factor '; and since the predictions envisage the rest of the
child's educational career and indirectly the rest of his working life,
this factor must have stability or permanence. We thus need
something like a first law of motion for mental activity: that is,
we require to distinguish a persisting internal * state of uniform
motion,' on which our predictions shall be based, from unfore-
seen ^c external forces,' which may subsequently come into action
and partly obscure or disturb it. Ideally we should like to detect
this permanent internal ^£ state' when the child first comes into
our ken, i.e. ^( at birth or at an early age.^7 Such a state, con-
ferred at birth and lasting throughout life, is precisely what is
meant by an innate capacity or propensity; and the external
forces are represented by environmental influences, such as training
at home, teaching at school, illness, accident, and the like.
Accordingly, from the very outset of my educational work it has
seemed essential, not merely to show that a general factor underlies
the cognitive group of mental activities, but also that this general
factor (or some important component of it) is innate or permanent,
in the sense just defined. In dealing with other tasks of psycho-
logical guidance—the treatment of delinquency or neurotic tendencies
or the subsequent choice of employment—similar issues arise : we