There are ethnologists who would not admit that such an explanation has anything to do with ethnology. They would contend that as soon as we cease to investigate the group as such we are passing from ethnology, the science of culture, to psychology, the science of individual minds. This seems an unnecessarily narrow doctrinaire view. Knowledge, as stated above, is not subdivided by hard-and-fast partitions. Interest certainly does not stop at an arbitrary point in the investigation but is centered on a comprehension of the _whole_ phenomenon. Where that phenomenon is an alloy of tin and copper, a decision as to its nature is naturally left to chemistry; it seems not unreasonable that where it is a type of association we should turn for enlightenment to psychology. Another field supplies an additional illustration. One of the important subjects for ethnographic study is artistic form. The ethnologist notes in a purely descriptive way the decorative patterns employed by various tribes, the fact that curvilinear motives are prominent among the Maori of New Zealand while the rawhide bags of Plains Indians are covered with angular paintings. Here, once more, it is clear that many of the problems that arise are purely cultural. There are, nevertheless, psychological elements involved that may be misunderstood without psychological knowledge. Let us assume, _e.g._, that a certain tribe is artistically characterized by a fondness for squares. What does this predilection signify? It is a psychological commonplace that through an optical illusion we exaggerate the height as compared with the width of a rectangle; accordingly, the geometrical square does not coincide with the psychological square. This simple piece of information enables us to understand _what_ we are actually dealing with in the case of a square pattern. At the same time it sharpens our observation regarding such patterns. It is quite conceivable that in one place tribal taste should prefer the actual square while elsewhere the psychological square occupies the seat of honor. This would be a purely ethnographic fact, yet its discovery might be considerably expedited by some knowledge of experimental aesthetics.