II. RESULTS OF QUANTITATIVE INVESTIGATION Many of the cherished hopes and desires of mankind concerning itself are in some part violated by the teachings of scientific psychology. Experimental psychology is not yet half a century old, dating its beginning as a technical science from the founding of Wundt’s laboratory at Leipzig, in 1879. Therefore, it is clear that the study of these problems by quantitative methods brings us very close to the present day. When the problem of measuring mental capacity was first taken into the laboratory, the modern definition of _a mental function_ began to be formulated. It became apparent that a mind must be judged by its product. The measurement of _performance_ is the only approach there is, or probably ever will be, to the measurement of mind. On this basis it was found impossible to identify or measure any such function as “the reason,” “the memory,” “the observation,” “the imagination,” “the will,” and similar supposed entities. _A mental function_ came to be defined as “_an actually or possibly observable event in behavior_.” Thus, memorizing digits, detecting absurdities, and reading English print are examples of mental functions, in the sense in which the term is used throughout the chapters of this discussion. Other terms which are used to refer to performances or “events in behavior,” are _abilities_ and _capacities_. A prolonged discussion might be conducted, in an attempt to assign different technical meanings to these words, and to bring out fine shades of distinction among them. For instance, it might be claimed that “ability” should be reserved to signify capacity _plus_ the skill acquired by practice, if any; while “capacity” should mean the innate aptitude, apart from all training. However, since capacity in this sense can never be known, but can only be inferred from the degree of actual performance, under controlled conditions, it hardly seems necessary to maintain such distinctions for our purpose. Refinements of nomenclature will, therefore, be avoided, and the terms _mental function_, _capacity_, and _ability_ will be used interchangeably, to denote performance which depends on the inborn integrity and sensitivity of the individual.