William Stern in his introduction to The Psychological Methods of Testing Intelligence (1914):

The objection is often made that the problem of intellectual diagnosis can in no way be successfully dealt with until we have exact knowledge of the general nature of intelligence itself. But this objection does not seem to me pertinent….We measure electro-motive force without knowing what electricity is, and we diagnose with very delicate test methods many diseases the real nature of which we know as yet very little.