HISTORICAL The science of high dilutions in historical context Silvia Waisse* Pontif  ıcia Universidade Cat olica de S~ ao Paulo, S~ ao Paulo, Brazil This paper begins from a discussion of the terms currently applied to the preparations used in homeopathic practice and relates them to the names given to them by Hahne- mann, the founder of homeopathy. The latter were directly related to the notion of mat- ter Hahnemann held, this is discussed as it evolved from the earliest sources to the sixth and final edition of the Organon of Medicine, through all the editions of this work as well as many other hitherto rarely explored sources. This review shows that Hahnemann moved conceptually within the field of concepts of late 18th- early 19th century science. This is only natural, and, a legitimate object for studies of the history of science and med- icine. Homeopathy (2012) 101, 129e137. Keywords: History of medicine; History of homeopathy; Hahnemann; Small doses; Theory of matter Introduction:What’s in a name? Research in high dilutions is increasingly recognized as an emergent scientific field. Currently, it is approached from a multidisciplinary perspective that includes physics, chemistry, biology, medicine, veterinary, botany, history and philosophy of science, and other disciplines. Although it has entered the university milieu, it has not yet been institutionalized as an autonomous discipline. Several properties have been described regarding the physical, chemical and biological behavior of such solutions. 1 How- ever, many aspects are still highly controversial, to begin with the most fitting term to name them. Samuel Hahnemann (1755e1843) e the founder of homeopathy and the first to formulate a therapeutic system grounded on the use of extremely small doses of medicines e prepared these dilutions through a double pro- cess of serial dilution and agitation of substances. He re- ferred to this process as ‘dynamization’ or ‘potentiation’, and the corresponding products ‘dynamizations’ (Dynami- sationen) or ‘potencies’ (Potenzen) since he believed that this procedure “developed dynamic latent powers [.] hid- den or slumbering” within the raw matter of remedies (Organon 6th edition x269). 2 As discussed below, such a conception corresponded to 18th century notions of matter that became obsolete by the mid-19th century. For this reason, several alternatives have been suggested e but are disputed e to replace those anachronistic terms, including: ‘infinitesimal doses’, ‘ultra molecular dilutions’, ‘ultra dilutions’, ‘ultra high dilutions’, ‘high dilutions’, ‘serially diluted and agitated’ substances, etc. None of them is entirely appropriate, because above the limit of Avogadro, according to current physicalechem- ical notions, there is no more ‘dilution’ of anything, since there is no more solute, although it is possible that ongoing research might change this assumption in the future. 3 A further issue deserving attention is strictly epistemo- logical: what should be the rationale for naming such prep- arations? Hahnemann’s terms alluded to their possible physical nature; the cognates of ‘dilution’ allude to the pro- cess of preparation and the molecular contents. The fact is that, as the nature and mode of action of these preparations are not explicable, nomenclature remains a task for the fu- ture. Since it also requires the agreement of the community of users, in this article we use the term ‘high dilutions’ (HDs), which results from a consensus by the members of the International Group for Research into the Infinitesi- mal (GIRI) to name the journal it publishes (International Journal of High Dilution Research). It is well-known that extremely small doses are one of the main distinctive traits of homeopathy, so much that popular language has incorporated the expression ‘homeo- pathic’ to refer to anything in minute amounts. Such small doses extend to inconceivable mathematical expressions: 1,000,000c corresponds to 1  10 2,000,000 , the more *PUC-SP/CESIMA, Rua Dr Caio Prado, 102, Sala 48, Consolac ¸ ~ ao, 01303-000 S~ ao Paulo, SP, Brazil. E-mail: dr.silvia.waisse@gmail.com, swaisse@pucsp.br Received 28 August 2010; revised 12 September 2011; accepted 30 January 2012 Homeopathy (2012) 101, 129e137 Ó 2012 The Faculty of Homeopathy doi:10.1016/j.homp.2012.01.001, available online at http://www.sciencedirect.com