While fully acknowledging the inseparable correlation between all psychical activities and the physical structures which condition them, let us not fall into the common and gross error of supposing that physical is in any way a measure of psychical function. All measurements in experimental psychology, be they by chemistry or physics, are quantitative only, and can be nothing else (Wundt); whereas psychical comparisons are purely qualitative. A single example will illustrate this infinitely important fact:—precisely the same quantity of physico-chemical change may be needed for the evolution into consciousness of two ideas; but if the one is false and the other true, their psychic values are indefinitely apart. We perceive, therefore, that in psychology generally, and especially in ethnic psychology, where we deal with aggregates, we must draw a fundamental distinction between those agents which act quantitatively on the psychical life, that is, modify it by measurable forces, and those which act qualitatively, that is, by altering the contents and direction of the _psyche_ itself. The former belong properly to “natural history,” and can be measured and estimated just to the extent that we have instruments of precision for the purpose; the latter wholly elude any such attempts, and must be appraised by the results they have historically achieved, that is, by arts, events, or institutions. The recognition of these two factors of human development, radically distinct yet inseparably associated, has led me to adopt the division into two parts of the present work. The first is the “natural,” the second, the “cultural,” history of the ethnic mind.[1] Footnote 1: The author had apparently decided to reverse this order of treatment after writing the above. The “natural history of the ethnic mind” forms the second part of the work.—EDITOR. Note that I say _ethnic_ mind. For let it be said here, as well as repeated later, that there is no such thing as progress or culture in the isolated individual, but only in the group, in society, in the _ethnos_. Only by taking and giving, borrowing and lending, can life either improve or continue. The “natural” history will embrace the consideration of those general doctrines of continuity and variation which hold true alike in matter and in mind, in the soul as in the body, and a review of the known forces which, acting through the physical structure and function upon the organs which are the vehicles of mental phenomena, weaken or strengthen the psychical activities. The “cultural” history will present something of a new departure in anthropology—a classification of all ethnologic data as the products of a few general concepts, universal to the human mind, but conditioned in their expressions by the natural history of each group. The justification of this procedure, which is _not_ a return to the ideology of an older generation, will be presented in the introduction to the second part.