|Top: a 2008 synopsis of Goethe and his 1809 chemical theory by English science dictionary editor John Daintith, from his Biographical Encyclopedia of Scientists.  RIght: an island model of the estate from Goethe's Elective Affinities, showing Edward, main reactant B, and the gardener (Gar), an intermediate system reactant, talking, before crossing the boundary (reaction membrane).|
A synonymous term to "human chemical theory", in reference to Goethe's work, is: Goethe’s human chemistry; a 2008 Hmolpedia-coined categorization term.
In 1796, German polymath Johann Goethe, in his Third Lecture on Anatomy (see: Goethe timeline), began discussing his newly forming theory that humans are types of chemicals, similar to reactive minerals, whose reactions to each other are governed or rather predetermined by forces external to the reactants themselves. He published the finalized form of his theory in his 1809 physical chemistry based novella Elective Affinities and up until his last day of existence (22 Mar 1832), or at least up until 20 Jul 1831, would go on to comment on this theory, which he said was his greatest work, in conversation, correspondence, and interactions, in various ways.
This theory, by Goethe, however, although greatly discussed and critiqued, remained largely unnamed in the two centuries to follow, at least in the English language; the German term wahlverwandtschaften may possibly embody Goethe’s theory to Germans and German-speaking people (see: title decoding). In English discussions, however, the rather ambiguous phrase “the chemical theory” or simply “theory” tends to be used.
|A 2011 German depiction of Goethe’s human chemical theory, by German blogger Peter Glaser, simply labeled as “Die Wahlverwandtschaften” (retouched by American electrochemical engineer Libb Thims: with English subtitle and thought bubble), which has a more complex meaning that what, in someone in colloquial English were to label as the “chemistry of love”, being that the German term Die Wahlverwandtschaften = chemistry of love + physics of love + thermodynamics of love, all in one phrase, something not easily rendered into English.  A similar term is “Elective Affinities Problem” the query that led to the human chemical theory.|
The following is a 1987 synopsis of Goethe’s human chemical theory according to American German-literature scholar John Winkelman: 
“One evening Eduard, Charlotte, and the Captain discuss a chemical reaction between two compounds each consisting of two elements. The compounds are designated as AB and CD. As a result of the reaction the elements form new compounds, AD and CB, and thereby demonstrate that they had a stronger ‘elective affinity’ to their new partners than they had to their original mates. The conversation turns to the issue of free will. Are the elements really inert or do they possess volition? The latter would seem to be the case, since the elements do after all choose one partner in preference to their other as if they were free agents. Charlotte dismisses this theory as idle speculation: one cannot equate chemical elements to human beings. But what about human beings? If the chemical compounds in the analogy had been human couples, would the partners have been forced to obey the ‘elective affinities’ that attracted them to new lovers or would they have had the freedom to honor their existing commitments in conformity to ethical law, which unlike natural law not only permits by implies free will?”
The following is a 2003 synopsis by William Burns, from his Science in the Enlightenment: and Encyclopedia: 
“[Goethe's] novel Elective Affinities employs a metaphor from chemistry—the ‘elective affinities’ were the different ways that different substances unite with each other—to describe human relationships. The characters engage in a lengthy discussion of current chemical theory.”(add discussion)
The following is a 2007 usage of the phrase “the chemical theory” by English German-cultures scholar Carl Krockel: 
“The chemical theory is the structural backbone of Goethe and George Elliot’s realism. Chemicals are only electively affined when their attraction excludes other chemicals. In his narrative Goethe uses the framework of two pairs of lovers, whose attraction to different aspects of each other reveals their psychological ‘properties’. Eduard and Charlotte are married, having known each other since childhood; the Hauptmann arrives, and links up with Eduard in their horticultural plans while excluding Charlotte for being too fanciful. Charlotte is satisfied by the arrival of the childlike Ottilie, and is also finding her own measured nature in affinity with the Hauptmann’s. Meanwhile, Ottilie appeals to Eduard’s childlike side, but the Hauptmann finds her ideas disturbing. And so the narrative continues. In Middlemarch George Eliot systematically elaborates on what Goethe only suggests in Die Wahlverwandtschaften, since each of her four characters is also bound to wider social relationships through affinity.”
In doing searches for key terms ‘Goethe’ and ‘chemical theory’ one will also come across these two terms discussed in the context of Goethe’s chemical theory of color, which published in 1810.
In reference to Winkleman use of the term "new lovers", above, see also: new relationship energy. 
In regard to the use of the term "analogy", above, to note, in 1982, American German-romanticism and humanities scholar William Lillyman, in his Goethe Symposium talk “Analogies for Love: Goethe’s Die Wahlverwandtschaften and Plato’s Symposium”, argued that Greek comedy playwright Aristophanes (446-386BC) had some type of precursory connectiveness to Goethe's human chemical theory: 
“Underlying the overt analogy for human love—or ‘Gleichnisrede’ to use Goethe’s term—and analogy which was taken from Aristophanes' speech in Plato’s Symposium. It not exaggerated to say that parallels and allusions to Aristophanes’ explanation of the nature of love permeate Goethe’s Die Wahlverwandtschaften.”
|A 2007 usage of the term “human chemical theory”, in reference to Goethe’s Elective Affinities, from American electrochemical engineer Libb Thims’ Human Chemistry. |
● Human chemical reaction theory
● Combination lock theory
● Human molecular hypothesis
● Goethean philosophy
1. Winkelman, John, (1987). Goethe’s Elective Affinities: an Interpretation (pg. 1). Peter Lang.
2. Krockel, Carl. (2007). D.H. Lawrence and Germany: the Politics of Influence (pgs. 21-22). Rodopi.
3. Burns, William E. (2003). Science in the Enlightenment: an Encyclopedia (pg. 116). ABC-Clio.
4. Daintith, John. (2008). Biographical Encyclopedia of Scientists (pg. 297). CRC Press.
5. Thims, Libb. (2007). Human Chemistry (Volume One) (human chemical theory, pg. 2). Morrisville, NC: LuLu.
6. New relationship energy – Wikipedia.
7. (a) Lillyman, William J. (1982). “Analogies for Love: Goethe’s Die Wahlverwandtschaften and Plato’s Symposium” (quote, pg. 128). Goethe’s Narrative Fiction: the Irvine Goethe Symposium. Walter de Gruyter.
(b) William J. Lillyman (faculty) – University of California, Irvine.
(c) Aristophanes – Wikipedia.
8. Glaser, Peter. (2011). “Die Wahlverwandtschaften” (The Elective Affinities), Glaserei Blog, Sep 9.
● Adler, Jeremy. (1990) “Goethe’s Use of Chemical Theory in His Elective Affinities”, in: Romanticism and the Sciences, ed. A. Cunningham and N. Jardin. Cambridge University Press.