: 
J.—PSYCHOLOGY. 175 
its angles equal, then it must needs also have all its sides equal. The 
second phase is to find out where, if at all, the conditions are actually 
fulfilled. Again, no assumption or hypothesis of any kind is involved ; 
the matter is nothing more than observation of fact. 
But then comes the third and last phase, that of supplying the factors 
with some plausible interpretation. Here the most cautious procedure 
and one that goes not an inch beyond what has really been proven, is simply 
to denote these factors by the non-committal letters g and s respectively. 
Any interpretation going beyond this can only, in our present state of 
knowledge, have a provisional value; it can serve to inspire further 
investigation, by which it will assuredly suffer much modification itself. 
With this reservation, then, the hypothesis at present seeming most 
helpful and suggestive is that the g measures something of the nature 
of an ‘ energy ’ derived from the whole cortex or wider area of the brain. 
Correspondingly, the s’s measure the respective efficiencies of the different 
parts of the brain in which this energy can be concentrated ; they are, 
so to speak, its ‘ engines.’ Whenever the mind turns from one operation 
to another, the energy is switched off from one engine to another, much 
as the power supply of a factory can be directed, at one moment to 
turning a wheel, at the next to heating a furnace, and then to blowing 
a whistle. 
Il. Recent Confirmation. 
So much to serve as a general description of the doctrine. I will now 
bring to your notice some recent work whereby its foundations have 
received additional strengthening, both on the side of mathematics and on 
that of actual observation. 
To take the former first, the earlier researches had only shown what 
conditions are necessary for the divisibility into the two factors. Later 
investigation has proved that these conditions are also sufficient. In 
other words, we now know, not only that under the said conditions the 
divisibility into two factors may possibly occur, but even that it inevitably 
must do so. I stress this point because some of the recent writing on the 
subject appears to make the contrary and mistaken statement that, even 
when the conditions are satisfied, still the divisibility either may or may 
not ensue. 
As to the precise nature of these conditions, they are based upon what 
are called the co-efficients of correlation. Such co-efficients, as is now 
generally understood, consist in numbers that indicate just how closely any 
two abilities or other characters tend to vary in proportion to one another. 
They are usually symbolised by the letter 7. Thus, 7,, would denote the 
degree that any ability 1 tended to vary from individual to individual 
proportionately to some other ability 2. Now, the conditions for the 
divisibility into the two factors reduce themselves to the simple equation : 
Pintle Tags Tipe 
Here the value on the left is called the tetrad-difference. When such 
tetrad-differences remain equal to zero throughout any set of abilities, 
whichever of them may be taken as J, 2, 3, and 4, respectively, then, and 
then only, each of these abilities can be divided up into two factors 
