The Enlightenment revolt against rationalism

Stephen Gaukroger, The University of Sydney

Peter Hanns Reill Vitalizing Nature in the Enlightenment, Berkeley, University of California Press 2005 (388 pp.) ISBN 0-52024-135-5 (hard cover) RRP $155.95.

The European Enlightenment is generally seen as a formative event in Western modernity, replacing religion, superstition, and absolutist monarchy with reason and democracy. Yet since the middle of the 20th century the shift of values that the Enlightment apparently brought with it has been increasingly questioned. In a 1946 essay, influential German social theorist Max Horkheimer complained of ‘the collapse of a large part of the intellectual foundation of our civilisation’. For Horkheimer, the collapse was due to ‘technical and scientific progress’, which he traced back to the Enlightenment, and above all to what he identified as the ‘self-destructive tendency of Reason’ characterising the Enlightenment mentality (1996/1946, p. 359).

Horkheimer was certainly not the first to trace the deleterious transformation of modern culture back to the effects of science. At the end of the 18th century, Edmund Burke had put the French Revolution down to an attachment to cold rational systems, advocated by radicals who ‘despised experience’. Forty years earlier, the French mathematician and encyclopaedist Jean le Rond d’Alembert had disparagingly referred to the path-breaking L’Esprit des Lois (1748) of Charles Montesquieu as ‘the spirit of Descartes applied to politics’, and complained that ‘our century seems to want to introduce cold and didactic discussions into things of sentiment’. Indeed, the phenomenon can even to be traced back to the very end of the 17th century, to Bernard de Fontenelle’s 1699 claim, in the Preface to his Histoire de renouvellement de l’Académie, that

The geometrical spirit is not so attached to geometry that it cannot be taken and applied to other knowledge. A work of morals, politics, and criticism, perhaps even of rhetoric, would be improved, other things being equal, if written by a geometer.

Once one probes below the surface, however, the values actually espoused by Enlightenment thinkers look a little less straightforward. In particular, it is difficult to discover just who is actually advocating the cold rational systems. Fontenelle was not describing an actual state of affairs but offering an intentionally provocative picture which corresponded to no one writing at this time: there was no one who pursued belles lettres in the geometrical spirit he described. Burke’s association of rational system-building and radicalism is complicated by the fact that his principal target, the English natural philosopher and non-conformist divine Joseph Priestley, was in fact one of the strongest opponents of system-building in the 18th century, regarding it as the source of all error, prejudice, and illegitimate authority.

It is difficult to discover just who is actually advocating the cold rational systems.

The case of d’Alembert is even more intriguing. Despite his criticism of Montesquieu, he was one of his greatest admirers, comparing him to Isaac Newton. Indeed, what he admired most was Montesquieu’s rejection of an approach that would impose a system upon the phenomena in favour of one that was sensitive to the great diversity in the phenomena. To make things even more complicated, in his Preface to the Encyclopédie, d’Alembert had attempted to save what was valuable in the systematic approach, arguing that reduction to as few principles as possible was crucial, and citing mathematics as especially valuable. But d’Alembert’s resignation from the editorial board of the Encyclopédie allowed the editor, Denis Diderot, in his De l’interprétation de la nature (1753), to rebut d’Alembert’s attempt at a middle way between rigorous deduction and observational exploration. Diderot called mathematics no more than a game, and predicted the imminent demise of the systematic deductive discipline of geometry in favour of the open-ended, empirical, exploratory disciplines of chemistry and natural history. Later, in Le Rêve de d’Alembert, Diderot pictures the sleeping d’Alembert forgetting his defence of systems and realising that the unsystematic vitality of a swarm of bees is the true model of natural processes (see Riskin 2002).

The twin questions of the viability of conceiving of nature in terms of a single hierarchical system, and of sensitivity to empirical diversity, figure large in 18th century thought, but not because they are two clear-cut opposing schools. Certainly, for polemical purposes (and writers like Fontenelle and Burke were inclined to subordinate everything to polemics), it was useful to present the contrasts that way. The reality, however, was much more complex, and it turned largely on specific questions such as the standing of chemistry and natural history vis-à-vis physics, the rationale behind botanical classification, the actions of muscles and nerves in physiology, as well as on general issues such as whether organic or inorganic models were appropriate for matter theory, for chemistry, or for economic and political theory.

In Vitalizing Nature in the Enlightenment, Peter Reill takes up the traditional contrast between Enlightenment mechanism and Romantic vitalism. He shows in some detail that mechanism was beginning to be replaced by vitalism as early as the middle of the 18th century. On the traditional view, when Enlightenment thinkers turned their attention to the ‘moral sciences’—which included political economy, civic education, law, private and public morality, and education—they explicitly modelled them on the natural sciences. But these natural sciences were themselves modelled on a mechanical world-view in which the only ultimate explanatory elements were matter and motion, so that vital phenomena were not recognised as such, and were accounted for in terms of the behaviour of constituent microscopic corpuscles.

Peter Reill takes up the traditional contrast between Enlightenment mechanism and Romantic vitalism.

It was realised that the perceived complexity of the world was hardly to be accounted for in terms of such limited explanatory resources. However, the response was not to attempt to amplify these resources, but rather to deny that the perceived complexity was a genuine feature of the world. In effect, only phenomena that could be accounted for in mechanistic terms were construed as being genuine: other phenomena were either merely secondary (such as the realm of sensibility: colours, smells, sounds) or, like vital phenomena, not genuine in their own right. The British natural philosopher Robert Boyle, for example, had argued in the 1660s and 70s that there is no essential difference between living and non-living things, and he defined life as a special case of motion or activity that arises from a specific organisation of corpuscles preordained in God’s original design of nature.

The approach Boyle exemplifies was widely seen as highly problematic throughout the second half of the 17th century. This was not least because there seemed to be an irreducible element of goal-directedness in organic processes: to understand them it was not sufficient to merely grasp the physical processes at work, one had also to grasp the functions realised by those physical processes, the goals or ends achieved by means of them.

During the 18th century Enlightenment, this concern grew into serious misgivings about the whole mechanist project, and it spilled over into the charged issue of over-systematic and rationalistic approaches to knowledge. By the mid to late eighteenth century, the key premise of mechanism, the idea that matter was uniform and inert, was coming under intense pressure. Mechanists had argued that until one had a mechanical explanation of natural phenomena, one had no explanation at all, but Newton, unable to devise a mechanist account of gravity, had concluded that the explanation must actually lie in other kinds of matter theory. If simple exchange of motion or momentum was not adequate to account for the physical changes that bodies induced in one another, the question was whether notions such as attraction and repulsion, loathing or desire, were an appropriate way of characterising what was happening in magnetic or chemical phenomena, for example. Of the three texts of the mid-century that set out a novel program in thinking about the natural order, the Encyclopédie, Montesquieu’s Esprit des Lois, and George Buffon’s Histoire Naturelle, it was the last that had the greatest immediate impact; and despite its great length at no fewer than 36 volumes, it was the third most commonly owned book in France by 1780.

Buffon argued that the basic properties that mechanists had ascribed to matter, and which they invoked as the ultimate explanatory principles—the impenetrability of matter, motion, external appearance, divisibility, communication of motion by impact, and the restitutive action of springs—are superficial. They are just the product of our way of seeing: if our senses had been different, so too would these have been. Using the model of gravity as a force underlying the sensible qualities of matter, Buffon argues that, were our senses able to penetrate beyond sensible appearances to the core of reality, what we would see would be matter as living or organic: life is the primary property of matter. Because matter is essentially alive, the categories of life should govern natural-philosophical enquiry. Inertness is not the essence of matter, as the mechanists had claimed. Rather, dead matter is a special case, and in this special case mechanism has some applicability but even dead matter exhibits a force, gravity, which it is beyond the resources of mechanism to account for. For Buffon, living matter is primary, and this primacy had methodological consequences: he rejects Linneaus’ influential botanical classification by visible parts (for example, leaves and fronds) on the grounds that these depend on superficial features and are mere formal abstract principles.

Chemistry was no longer an adjunct to a mechanical philosophy of matter, but an autonomous discipline.

Above all, history plays a key role in Buffon’s classificatory system: natural history must account for how a species has emerged from historical predecessors, and Buffon assumes that every species must have some sort of primitive model responsible for both its internal and external features, and providing a blueprint for all individuals. There are parallels here with Montesquieu’s model for the study of nations, in that, like an organised body, the nation’s spirit must be considered as a whole, for it is formed by a complex set of interacting elements. The search for understanding on the basis of things that are independent and self-evident was misguided on this view: comparison of similarities and differences is the basis on which progress in our knowledge of the world is to be made, although the use of analogy rises above mere comparison, establishing connections between form and force, structure and causal process. We are able to establish such connections only on the basis of giving up the notion of the scientist as a detached observer. For Buffon, observers are not independent but part of the natural processes that they observe. Indeed, it is in virtue of this that their very understanding of these processes is possible.

The growing dissatisfaction with mechanism as a model for physical processes prompted the question of what would form an adequate model, and the traditional discipline of natural history, as well as the new one of chemistry, began to play key roles. The inadequacy of mechanics as a general account of physical reality had been evident to Newton in his speculations on the nature of gravity, and he had adopted a chemical approach to revealing what he believed were natural forces such as gravity which operated at the microscopic level. By the second half of the 18th century, there was widespread dissatisfaction with traditional discussions of matter in terms of infinite divisibility, the nature of dimensionless mass points, etcetera.

Attention shifted to a very different kind of divisibility of matter, whereby matter was decomposed not into identical corpuscles but into a variety of constituents, each with different chemical qualities. Chemistry was no longer an adjunct to a mechanical philosophy of matter, but an autonomous discipline. The rationale behind this, as Reill points out, was fourfold: (1) different bodies mix with one another so intimately as to be beyond separation by mechanical means, and this chemical bonding exhibited an ‘internal fusion’ manifestly beyond the explanatory resources of mechanism; (2) chemical processes between substances having different qualities form other substances with essentially unique properties: two of the most flammable substances, hydrogen and oxygen, as Lavoisier discovered, combined to produce something that was guaranteed to extinguish fire—water; (3) ‘mixed’ substances can be decomposed only to a certain point, where they yielded not homogenous matter but a limited number of base or heterogeneous elements; (4) chemical combination and recombination seemed to be ruled by affinities between substances that fell outside the scope of mechanical explanation.

Connected with these insights was an even more fundamental shift in the basic assumption of matter theory. Matter was no longer considered essentially solid but fluid. Changes of state came to occupy the attention of chemists, above all the question of just what happened in the transition from a solid to a liquid state. Again the explanatory resources of mechanism were insufficient here, and the idea emerged that during the change of state, the solid became penetrated by an active subtle fluid, which released the substance from its cold imprisonment.

Indeed, matter came to be conceived as being in essence something continuously active that experienced countless transformations that proceeded at different rates and different temperatures depending on the specific qualities of the substance. If the new chemistry borrowed its language and images from another discipline, it was no longer physics but primarily natural history. In particular, the notion of equilibrium that came to play such a significant role in conceiving of how chemical processes were regulated, was not the static equilibrium of the mechanists, now associated with stagnation and death, but an ‘economy of nature’ involving a harmony arising from interplay between active forces, and between living and dead matter.

We find parallel concerns about the explanatory power of mechanist models in physiology and in political economy.

We find parallel concerns about the explanatory power of mechanist models in physiology and in political economy. In 1751, in his Essay on Vital and Involuntary Motions in Animals, Robert Whytt denied that animal systems could be modelled on machines because mechanical cause and effect could not explain the fact that, when touched, muscles contract with a force much greater than the original cause. Indeed, more generally, vitalist physiologists noted that physiological effects exceeded their immediate causes in terms of duration, intensity and extent. Not only could impulsion and transferred motion not account for the disproportion between cause and effect, but the mechanist emphasis on sequential change made it impossible to deal with the simultaneous reactions of different organs to a single stimulus.

Vitalists considered that the ‘animal economy’ needed to be conceived as a system consisting of animated matter organised around individual centres of life that worked together, thought through in terms of mutual ‘reciprocity’, ‘sympathy’, and ‘consensus’. What we have here is no longer a ‘system’ in the sense of a top-down hierarchically ordered structure, but rather a set of contingent relations between facts discovered by procedures such as induction and analogy. The relations involved are vital relations, mirroring the elective affinities of chemistry and the role played by gravity in dead matter: irreducible active principles.

The more general cultural impact of these debates is nowhere more evident that in political economy. The Physiocrats maintained that the understanding of nature was not limited to the physical sciences, but also encompassed the moral sciences. Anne-Robert-Jacques Turgot, not least in the entries on political economy that he wrote for the Encyclopédie, and François Quesnay urged that nature was not to be modelled on matter in motion because common experience showed that motion itself was always preceded by an act of will, so a proper understanding of nature must always be goal-directed. Similarly with that part of nature that made up political economy: we must comprehend nature’s ways and not, as the Mercantilists had done, attempt to impose an artificial system of complex taxes, tariffs, and regulation on it. Thus began trading of models between economic theory and physical theory, which came to a head in the swapping of models of social and thermodynamic equilibrium in neo-classical economics in the 1870s (Mirowski 1989). The integration of social and the physical models in the understanding of nature was, in short, something whose origins lay not in 19th century thought, but can be traced back to the Enlightenment.

Contrary to a traditional ‘rationalist’ picture of the Enlightenment, what emerged from the middle of the 18th century—the heart of the Enlightenment—was an anti-system ideology that took its model from a purpose-driven organic realm. Reill’s treatment of the Enlightenment development of organic models reveals a continuity with Romanticism that has been little suspected until recently.


Horkheimer, M. 1996, ‘Reason against itself: Some remarks on Enlightenment’, in What is Enlightenment? Eighteenth-Century Answers and Twentieth-Century Questions ed. James Schmidt, University of California Press, Berkeley, pp. 359–67.

Mirowski, P. 1989, More Heat than Light: Economics as Social Physics, Physics as Nature’s Economics, Cambridge, Cambridge University Press.

Riskin, J. 2002, Science and the Age of Sensibility, Chicago, University of Chicago Press.

Professor Stephen Gaukroger is in the Department of Philosophy at the University of Sydney. His most recent book is Descartes’ System of Natural Philosophy (2002, Cambridge University Press). His current research projects are ‘The Emergence of a Scientific Culture in Early-Modern Europe’ and ‘The Persona of the Philosopher’. In 2004 he began a 5-year ARC Professorial Fellowship.