By the end of the 17th century, natural philosophy had made an important shift toward adopting an orderly conception of nature. Scientific societies started to challenge wonders by questioning the truth of strange phenomena: the quest for truth and the norm was becoming incompatible with the love of the marvelous and unique. As a result of these changes, learned sensibilities became increasingly distant from their lay counterparts. Another consequence was that teratological research became increasingly specialized, focusing on the medical fields of anatomy and embryology.

Although wonders had lost their aura, the monstrous was the topic of several significant debates in the scholarly community during the 18th century. Following a controversy about the relationship between God and Nature, the scientists of the French Académie Royale des Sciences debated about the responsibility of God in the production of unusual or monstrous births. By the middle of the 18th century, Enlightenment philosophers shifted the terms of the debate and boldly set forth theories that presented the monstrous as a non-metaphysical phenomenon.

Throughout the 18th century, both philosophers and scientists, however, worked without recognized anatomical standards to guide them in the classification of monstrous births.

Académie Royale des Sciences. Histoire de l’Académie royale des sciences … Année 1740 …Paris: Imprimerie Royale, 1742.

Contrary to the Royal Society of London, the Académie Royale des Sciences was fully supported by the government, and therefore was a formal organization with professional scientists. The Histoire de l’Académie royale des sciences was a publication of the Académie. It provided commentary by the Académie’s Secretary on selected papers, and a synthesis of current research.

This report on monsters, written by the Secretary, summarized one of the most controversial questions of the French Academy concerning the origin of monsters: What was the origin of the foetus, and the cause of the evolution of the egg-foetus during its stay in the womb, such that it led to a monstrous birth?

Three physicians were particularly involved in this investigation. Joseph Duverney, a professor of anatomy in Paris, had described a case of conjoined twins and defended the theory that these were a demonstration of divine fecundity and variety. For him, monsters existed in the egg-foetus. On the other hand, Duverney’s opponent, the physician Nicholas Lemery, supported the theory of accidental origins, and believed that conjoined twins were caused by the accidental collision of two eggs. Jacques Winslow entered the debate in 1735 and tried to present a compromise by suggesting that, in certain cases, accidental causes could mask metaphysical forces.

Nicolas Malebranche (1638-1715). Tractatus de inquisitione veritatis … Geneva: Fratres De Tournes, 1753.

Malebranche was a priest and a member of the Académie Royale des Sciences. His book, The search for truth, went through numerous editions, in both the vernacular and in Latin, through the end of the 18th century. His work popularized what was termed the “imaginative” theory of the cause of monstrosities.

Malebranche theorized that monstrous births were caused by the imagination of the mother on the foetus, which was achieved through communication between the nerves of the mother and the nerves of the foetus. For example, a monstrous birth might result if a pregnant woman saw a particularly ugly or frightening object. Malebranche based this on the 16th century belief that imagination had the power to affect the outside world.

James Augustus Blondel (ca. 1666-1734). The power of the mother’s imagination over the foetus examin’d. In answer to Dr. Daniel Turner’s book, intitled A defence of the XIIth chapter of the first part of a treatise, De morbis cutaneis ... London: John Brotherton, 1729.

Blondel’s text was the first scientific work to refute the role of the imagination of pregnant women in creating monstrous births. More generally, although this belief remained popular, it was gradually rejected from aesthetics, natural philosophy, and medicine in the eighteenth century.

Blondel used the regularity of nature and its mechanical functioning as a ground against his opponents. He illustrated this with numerous clinical examples. Blondel also rejected the “imaginative” theory of monstrous births on the basis that belief in the power of the imagination was an unscientific idea held by lay people.

Guillaume Lamy (fl. 1650-1682). Discours anatomiques ... Rev. & augm. de toutes les plus curieuses decouvertes des anatomistes modernes … Brussels: & se vend a Paris, Chez Laurent d’Houry, 1685.

The Discours anatomiques was first published in 1675. In these public lectures, the French physician Lamy dealt with the nature of medical science. His search not for “final” or ultimate causes, but simply for the “efficient and material cause” led him to abandon any consideration of the divine role in the creation of unusual births. For Lamy, if we understand the world as fortuitous, then what we termed the “monstrous” and the “normal” were equally the products of natural formation.

Lamy’s nontheistic hypothesis had a significant influence on Enlightenment philosophy.

Denis Diderot (1713-1784). Lettre sur les aveugles: a l’usage de ceux qui voyent … London: [s.n.], 1749.

In his “Letter on the blind,” Diderot presented the issue of monstrosity as a strictly nontheistic problem.

Diderot belonged to the small group of French Enlightenment materialists who were influential in removing divine considerations from scientific understanding. He was incarcerated in the Bastille for three months following publication of this book.

Jacques Louis Moreau (1771-1826). Description des principales monstruosités dans l’homme et dans les animaux précédée d’un discours sur la physiologie et la classification des monstres … avec figures coloriées par N.F. Regnault … Paris: Fournier, 1808.



This book was originally published in 1775 by the artists Nicolas-François and Geneviève Regnault under the title Les Ecarts de la nature ou recueil des principales monstruosités (The Deviations of Nature or a Collection of the Main Monstrosities). For the first time in print, the artists, well aware of the susceptibilities of their readers, exploited the aesthetic beauty of monsters. The prospectus quoted the French poet Boileau: “no monster exists that cannot be made pleasing through art.”


Moreau de la Sarthe, the editor of the second edition, targeted a more scientific audience and added a 15-page introduction to the work that included the various sources of monstrosity, as well as a classification system of monstrosities.