This book does not claim to state all the laws of sexual affinity, for there are many; nor does it pretend to be able to tell every one exactly which individual of the opposite sex will best suit his taste, for that would imply a complete knowledge of all the laws in question. In this chapter only one of these laws will be considered--the law which stands in organic relation to the rest of the book. I am working at a number of other laws, but the following is that to which I have given most investigation, and which is most elaborated. In criticising this work, allowance must be made for the incomplete nature of the material consequent on the novelty and difficulty of the subject. Fortunately it is not necessary for me to cite at length either the facts from which I originally derived this law of sexual affinity or to set out in detail the evidence I obtained from personal statements. I asked each of those who helped me, to make out his own case first, and then to carry out observations in his circle of acquaintances. I have paid special attention to those cases which have been noticed and remembered, in which the taste of a friend has not been understood, or appeared not to be present, or was different from that of the observer. The minute degree of knowledge of the external form of the human body which is necessary for the investigation is possessed by every one. I have come to the law which I shall now formulate by a method the validity of which I shall now have to prove. The law runs as follows: “For true sexual union it is necessary that there come together a complete male (M) and a complete female (F), even although in different cases the M and F are distributed between the two individuals in different proportions.” The law may be expressed otherwise as follows: If we take μ, any individual regarded in the ordinary way as a male, and denote his real sexual constitution as Mμ, so many parts really male, plus Wμ, so many parts really female; if we also take ω, any individual regarded in the ordinary way as a female, and denote _her_ real sexual constitution as Wω, so many parts really female, plus Mω, so many parts really male; then, if there be complete sexual affinity, the greatest possible sexual attraction between the two individuals, μ and ω, (1) Mμ (the truly male part in the “male”) + Mω (the truly male part in the “female”) will equal a constant quantity, M, the ideal male; and (2) Wμ + Wω (the ideal female parts in respectively the “male” and the “female”) will equal a second constant quantity, W, the ideal female.