IT appears desirable to utilise the remaining available pages for a statement of the present position of the controversy which forms the subject of the two preceding chapters, especially as in discussion points of difference are apt to be magnified and points of agreement lost sight of.

The term “general intelligence” or “general ability” is liable to have two distinct meanings. On the one hand it may be a statement of a fact, on the other an explanation of that fact. The fact which makes the term a necessary and a useful one is that a man who is good at one kind of mental work is usually above the average in others. In technical language, most measures of correlation between various mental tests, or between various school and university subjects, are positive, and many are high. Though some are low, few are negative. When this is denied, it is generally on the strength of a number of individual cases where marked ability is found in one subject but not in another. These are, however, swamped by the much larger number of cases in agreement with the principle. Because of this fact of predominant positive correlation, it is possible, after administering an intelligence test lasting one or two hours, to predict an individual's performance in various mental activities with more or less probability, though never, of course, with absolute certainty. If the known correlation between the test and a certain other activity is r, then an individual who deviates d from the average in the test (in sigma units) will deviate rd from the average in that activity most probably. In practice however such individuals who deviate d in the test will not all be exactly at rd in the other activity, but will be scattered about it. And that scatter will be less than the scatter of an unselected group in the proportion k : 1, where k= √ (l — r2). The test by its constituent elements probes the mind at a number of different points and strikes an average, just as one finds the depth of a lake by plumbing it at various points.