In modern times, it is of course known that smells are manifest by chemical processes in which the molecules of the smell activate specific nerves which in turn excite certain areas of the brain. However, as little as fifty years ago, it was thought, by aromatherapists especially, that smells function by means of waves of energy characteristic of the source of the smell, and thus the inherent healing power of the smells. Some 2500 years ago, the world was thought to be comprised of the four basic elements of air, fire, earth, and water, which correspond to the four humours and several other similar descriptors of the natural world. These elements also corresponded to relative levels of temperature and moisture which accurately depicted an individual, ranging from hot and wet to cold and wet. If an individual allowed himself to meander too far in any direction, his treatment consisted of the proper amount of the opposite element to balance his temperament. This remains to be one of the best illustrations of the method of perfume design. Many perfumes are designed with a "season" in mind, which easily relates to the four elements and also the four personality types, and many people naturally choose a perfume that either accurately depicts their personality or one that is the exact opposite.
A necessary evil in the perfume industry is the ever elusive trend. The very nature of trends make them almost impossible to study, but certain characteristics of trends seem to be constant. Trends usually begin with what is called the gag phase, or experimental phase. At this point, most trends are jokes. From here, trends assemble themselves into what is called the signal phase, which consists mainly of groupings of gags. Although it is usually thought that trends come into being by sheer number of repetitions or audience appeal, it is rather a common historical thread that tends to form a trend. These historical threads usually have less to do with what has actually occurred, but rather an aspect of society which is either readily apparent or deeply hidden, which the trendsetters wish to bring out. From this stage, trends rapidly mature and die, with the life span usually lasting from three to nine months for perfumes. Periods characterized by numerous new perception structures are almost always followed by a nostalgia wave, which tends to take the form of a classical revival, and which is brought on by both jaded ideas and market saturation. In perfumes, most trends are set by teenage girls willing to experiment or women over the age of forty who have satisfied their family obligations and are entering a stage of occupational and personal self-realization. Over the past decade, an increasing awareness of the condition of the environment has resulted in an infatuation with perfumes containing a dominant citrus note, while very recently a concern with a decline in morals and obligation has changed the focus to "floriental" perfumes, characterized by their beautiful and calming fragrances. However, an overemphasis on nature over the past several years is now producing a resurgence in totally synthetic perfumes, evident in the freshly launched campaign for Chanel No. 5.