THE RATIONALE OF ODOR CLASSIFICATION* By ERNEST C. CROCKER A•rthur D. Little, Inc., Cambridge 42, Mass. AT THE TECHNICAL meetin g of THE SOCIETY OF COSMETIC CHEMISTS held in May of this year the subject of odor classification came up for dis- cussion. Mr. Edward Sagarin voiced objection to the consideration for adoption ooe any existing system of odor classification on the ground of what he has termed: "inherent in- validity." That viewpoint had the effect of being a challenge to the Crocker-Henderson system of odor classification and the least that one of its sponsors could do was to meet that challenge. The present pair of papers is the result. It is my sincere hope that this presentation ooe two sides of a controversial subject will prove enlightening for the members of the SOCIETY and also will ad- vance effective odor classification. For my part, I welcome this ex- change of ideas for it has brought about an objective analysis of the system which might not otherwise have been made. Both of us agree on many points. To save time, we both see the great desirability of odor classification provided it can be done on a sound * Presented at the December 8, 1949, Meeting, New York City. basis, and we are both talking about qualitative characterization and not about thresholds of intensity or other matters of strength or quan- tity of odor such as may be measured by an osmoscope. We agree on the existence of what the psychologist calls "pure odor," as against odor sensations which involve trigeminal components including those of pain, which is called "pungency," and of coolness, warmth and contact. We further agree that no present theory of odor or smelling is soundly enough established to be the basis of a system of odor classification. What we disagree on is the possibil- ity of obtaining a workable system of odor classification in the absence of a valid theory of odor. From the standpoint of logic, it is desirable to have a firm foundation before we build a structure of any kind, on the analogy that without sound premises we cannot arrive at valid conclusions. There is no dis- agreement with this procedure in rhetoric, or mathematics, or in law, but often we are forced to use other techniques in laying the foundations of the physical sciences. Some- times, pure assumptions are first

16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS made and tested, and if the findings are correct, then something of the nature of the underlying laws and facts can be discovered. This is a kind of logic in reverse. Again, some physical facts, such as gravita- tion, matter, electricity, and mag- netism, are so difficult to under- stand that we can treat them only as mathematical concepts until enough of their properties have been ob- served that we can adjust our minds to conceive of them as real, even though incomprehensible to our senses. Sometimes the finally ac- cepted fact seemed illogical right up to the time that we were forced to accept it. Sir Isaac Newton sought valiantly to choose between the only two theories of his time on the nature of light. The wave theory accounted for some facts and the corpuscular theory accounted for others. Neither theory was ade- quate to account for all. These two theories, on any physical basis, were mutally antagonistic and incom- patible and hence could not be com- bined. After some three hundred years, however, it has been found that light can be described as both corpuscular and wave-like, even though most physicists still cannot visualize a physical reality that can combine these two attributes. We do not know anything finally, completely, or fully, yet we do fairly well in utilizing matter, energy, and the associated radiations. Some- times, using even a wrong hypoth- esis, we are led to an approach close enough to the underlying truth that with a few more at- tempts we find it. Columbus had the wrong idea about where he was going, but the result of his voyage was most useful to geography and the world. The point was that he did something, and did not just con- clude that until detailed maps were in existence it would be of no use to go on a long sea trip. My feeling is that a system of odor classification which does not depend upon any special theory of odor may not only be useful in itself, but may even lead to the discovery of the nature of odor and smelling. Let us say that it is an attempt to find the cause through a careful study of the clues. There are literally hundreds of thousands of recognizably different odor sensations, or, as they are com- monly called, "odors." We shall use the term "odor" to mean the im- pression received on the sniffing of scented air. The aim of this paper is to show the inherent possibility of odor classification as such, of the utility of the so-called Crocker- Henderson system of odor classifica- tion, and to consider ways and means for the further improvement of this system and its operation. Odor sensations are carried to the brain by means of a nerve system from the odor-detecting area located high in the nasal chamber. We can conceive of two broad patterns of complexity of nerve-bundles for the conveying of the odor message: one, with a nerve or group of nerves responsive to and capable of con- veying each separate sensation, whenever stimulated, and the other,