Passion

 An iconic sexual encounter rain scene, from the 2010 film Young Goethe in Love, of Goethe’s passionate affair with Charlotte Buff—which was quickly extinguished owing to her family’s financial situation—and his other various intellectual and poetry “passions”, in the 1771, when Goethe was age 22, the year he became a lawyer, which may well capture one aspect Goethe's famous retrospect 1830 comment: “I lived every word of my Elective Affinities.” [7]
In terminology, passion refers an intense, driving, or overmastering feeling or conviction; a strong liking or desire for or devotion to some activity, object, or concept; or in the plural sense, as in "passionate", a state of intensely-felt emotion. [1]

Goethe
In 1799, German polymath Johann Goethe, in a letter to his intellectual friend German author Friedrich Schiller, commented the following on French author Prosper Crebillon (1674-1762), in regard to the lack of realism in his work:

Crebillon … treats the passions like playing cards, that one can shuffle, play, reshuffle, and play again, without their changing at all. There is no trace of the delicate, chemical affinity, through which they attract and repel each other, reunite, neutralize [each other], separate again and recover.”

Turned around in the form of a learned maxim, this comment would translate as:

“Never treat the passions like playing cards.”

which may very well capture the quintessential essence of Goethean philosophy. In loose equation form, Goethe conceptualized passion as function of individual attractions and repulsions mediated largely by the external forces of the elective affinities:

where A is chemical affinity or elective affinity (as described by Torbern Bergman). In short, Goethe is commenting here that Crebillon’s work is not realistic in the sense that it is not based on the way that people "react" to each other—just as chemicals react to each other—according to the principles, laws, and outcomes of physical chemistry.

Goethe, here, to note, in his "without their changing at all" comment, seems to be digging around at what would eventually come to be known as the "irreversibility" of nature, as captured in Greek flux-and-fire philosopher Heraclitus' motto that we "never step in the same river twice" and what German physical chemist Rudolf Clausius eventually came to quantify by the notion of entropy increase. Goethe, in any event, ten years later, would go on to work out the details and repercussions of this dense statement, in the form of a layered and coded physical chemistry based novella: Elective Affinities (1809), which thus officially-launched the science of human chemistry. The following, to exemplify the way in which "passion" is studied in the novella, is an 1859 synopsis of Goethe’s novella from the New American Cyclopedia: [8]

“In 1809, Goethe printed the most exceptionable of his novels, the Wahlverwandschaften (“Elective Affinities”), in which the charms and graces of this style are employed in the description of the impulses which spring from the collision of passion and duty in the relations of marriage. By the title of the book, and in the whole spirit of it, he would represent that sexual affinities follow the same inevitable law as chemical affinities, and that humanity struggles impotently against the dictates of nature. Like all his productions, this was suggested by circumstances in his own experience. The work shocked the moral world, in spite of the beauty with which it was written, and to this day tasks the ingenuity of those of his admirers who seek to defend it from attack.”

In P2:C4, Goethe ends by giving the following reflective maxims on passion, which he lists as entries to Ottilie's diary:

“The passions are defects or excellencies only in excess.”

“Our passions are true phoenixes: as the old burn out, the new straight rise up out of the ashes.”

“Violent passions are incurable diseases; the means which will cure them are what first make them thoroughly dangerous.”

Passion is both raised and softened by confession. In nothing, perhaps, were the middle way more desirable than in knowing what to say and what not to say to those we love.”

Thermal theory of affinity | Thermodynamic theory of affinity
The term "passion", as employed historically in literature (see: literature chemistry and literature thermodynamics) tends to be representative—as a thermal word—of the physical term "heat" and, in this sense, following the great 1854 to 1882 “thermal theory of affinity debate”, on the topic of whether or not heat was the true driving force of chemical reactions, in the sense that the more heat that was released in a given reaction the greater the affinity between the reactants, the chemical interpretation of passion and change only become more complicated. The "heated" debate, however, was quenched when in 1882 German physicist Hermann Helmholtz published his chemical thermodynamics-launching article "On the Thermodynamics of Chemical Processes" in which he proved that "free energy" (not heat) is the true measure of chemical affinity and wherein he gave the following formulation for chemical affinity:

meaning that the affinities A are "active" only in the sense in which the free energy differentials dF of the reacting system decrease with differentials of time dt. The century to follow saw purifications of this logic, significantly in the works of: Nernst (1906), Lewis (1923), De Donder (1936), and to some extent Perrot (1998) in his dictionary descriptions of the relations of affinities and free energies (see: equation decipherment):

 Equations of Elective Affinities $A = - \Delta U \,$ $A = - \Delta G \,$ $A=-\left(\frac{\partial G}{\partial \xi}\right)_{p,T}$ $A^{\circ} = \sum_{i=1}^k - \nu_i \mu^{\circ}_i \,$ Helmholtz (1882) Nernst(1906) Lewis(1923) De Donder(1936) Perrot(1998)

In short, the human interactions of each chapter of Goethe's novella need to be explained in terms of partial differential equations of Gibbs free energy or human free energies per extent of reaction; or in terms of components, in the Lewis-sense of the affinity formulation:

$A = - \Delta G \,$

with substitution:

$A = - ( \Delta H - T\Delta S) \,$

with carry-through of the negative sign:

$A = T\Delta S - \Delta H \,$

the "delicate affinities" Goethe spoke of in 1799, in modern terms, need to be upgraded to the "delicate entropies" ΔS and "delicate enthalpies" ΔH involved in the reactive processes through which people, in Goethe's view, "attract and repel each other, reunite, neutralize [each other], separate again and recover.” In short, with the 1882 Helmholtz disproof of the thermal theory of affinity, putting in place of it the "thermodynamic theory of affinity", namely that the true driving force of chemical reactions is not "heat" but rather "free energy"—Gibbs free energy in the case of freely-going isothermal-isobaric surface-attached human chemical reactions—the modern literary writer or social theorist, in the years to follow, would have needed to reinterpret "passion" in terms of not heat but free energy, in particular Gibbs free energy, if one was to construct a drama, literary work, or social theory using chemical models.

In 1896, French-Italian mathematical engineer Vilfredo Pareto, in his Course on Political Economics, supposedly aware of the new chemical thermodynamics upgrade, via his knowledge of the work of American engineer Willard Gibbs, defined society as system of human molecules in a complex mutual relationship”, and further posited that each human molecule (person) only acts in response to the force of ophelimity—a formulation that supposedly removes (or deanthropomorphizes) the notion or concept of passion from the description of man:

“First we separate the study of ophelimity (economic satisfaction) from the diverse forms of utility, then we direct our attention to man himself; stripping him of a large number of his attributes, leaving out the passions, good or bad, reducing him to a kind of molecule that only acts in response to the forces of ophelimity.”

In 1914, English-born American chemical engineer William Fairburn, in his Human Chemistry, defined people—similar to Goethe (although he does not cite Goethe)—as “human chemical elements”, and argued that people, as chemicals, could not only be classified by their reactive affinities among each other, but also by their respective "entropy" measure (entropy being a component of Gibbs free energy), which he says can be likened to passion: [3]

“A classification based on their relative electricity or relative energy or enthusiasm would not of itself help us much, for misapplied energy and wasteful application of human forces are common. The classification of entropy, referring to temperature changes which can be likened to coolness, passion, explosiveness and frigidity, are all interesting but of themselves prove little.”

In 1993, British playwright Tom Stoppard debuted his Elective Affinities rewrite stage play Arcadia, wherein he incorporates talk of sexual energy, heat, entropy, the second law, human chemical affinity, via his talk of “the attraction that Newton left out … all the way back to the apple in the garden” (see: Query 31), the steam engine, among other topics. [4]

In 2002, German chemist Volker Wiskamp, in his online article “Chemistry in the Work of Goethe”, discussed Goethe's Elective Affinities in terms of both enthalpy (enthalpy of formation) and entropy (entropy increase) which is a very rare find, as far as human free energy theorists go. [5]

In 2007, American electrochemical engineer Libb Thims, in his two-volume Human Chemistry textbook, expanded on the work of Goethe, commenting, in introducing the etymology of the term enthalpy, that: [6]

“In human intimate terms, enthalpy correlates to the heat of passion within human relationships.”

Discussion
Of note, in comparing Thims' 2007 view (enthalpy correlates with passion) with Wiskamp's 2002 view (enthalpy of formation correlates with passion) with Fairburn's 1914 view (entropy correlates with passion), one notices a but if inconsistency, in regards to interpretation of "passion" in chemical thermodynamic terms—which is a puzzling phenomenon, to some extent? This interpretation inconsistency, by comparison, is similar to Russian physical chemist Georgi Gladyshev's 2006 view that entropy is very small and nearly insignificant in social reactions, but supposed large and significant in laboratory chemical reactions—which is the opposite of German physical chemist Wilhelm Ostwald's circa 1910 view that entropy is large in social situations, but small in beaker-sized chemical reactions situations.

Quotes
The following are other noted quotes concerning passion:

“How few men reflect, and even among those who pay attention to themselves, scarcely any have found the thread for the labyrinth of our passions.”
Bernard Mandeville (c.1714) [9]

“The only unions which are legitimate are those ruled by a genuine passion.”
Stendhal (1822), On Love

Passion of Osiris
● Passion of Christ

References
1. Merriam-Webster Collegiate Dictionary (2000).
2. (a) Winnett, Susan. (1993). Terrible Sociability: the Text of Manners in Laclos, Goethe, and James (pg. 220). Stanford University Press.
(b) Lynch, Sandra. (2005). Philosophy and Friendship (Crebillon, pg. 37). Edinburgh University Press.
(c) Steer, Alfred G. (1990). Goethe’s Elective Affinities: the Robe of Nessus (Crebillon, pg. 37; symbolically, pg. 158). Winter.
3. Fairburn, William Armstrong. (1914). Human Chemistry. The Nation Valley Press.
4. Stoppard, Tom. (1993). Arcadia (heat, 6+ pgs; sexual energy, pg. 33; atom, 2+ pgs; second law of thermodynamics, pg. 65; quote: "Ah. The attraction that Newton left out. All the way back to the apple in the garden", pg. 74). London: Faber and Faber.
5. Wiskamp, Volker. (2002). “Chemistry in the Work of Goethe” (“Chemie im Werk Goethe”), University of Applied Sciences Darmstadt, Department of Chemistry and Biotechnology.
6. (a) Thims, Libb. (2007). Human Chemistry (Volume One) (passion, pg. 82). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two). Morrisville, NC: LuLu.
7. (a) Young Goethe in Love (overview) – MoveGuy247.com.
(b) Young Goethe in Love – Wikipedia.
8. Ripley, George and Dana, Charles D. (1859). The New American Cyclopedia: a Popular Dictionary of General Knowledge, Volume 8 (pg. 337). D. Appleton and Co.
9. Zinsser, Judith. (2006). Emilie Du Chatelet: Daring Genius of the Enlightenment (pg. 59). Penguin.