"The question first arises "what is a prostitute ?" To this the law answers, that it is one, who openly and with little or no distinction of persons, sells her favors for money : and who with this object endeavours to make herself publicly known as a prostitute. On the contrary, the woman, who does not court notoriety, but admits few lovers and in secret, although she receive money, cannot, and dare not, under penalty of damages for libel, be called a prostitute. This distinction is in Paris of great importance, for the police of that city exercise a surveillance over all the public prostitutes, who are obliged to eurol themselves in a registry, to receive sanitary visits &c., while they have no control over any other women. Hence the numbers, habits of life, and destiny of the prostitutes are much better known in Paris, than in any other city : and this gave M. Duchatelet facilities for gathering information, which he could have had nowhere else..."
George Drysdale. The Elements of social science… 1861