*This is part of an on-going series of articles about largely unknown Christians who had an enormous impact on society by faithfully living out their biblical worldview in various areas of life.*

The Enlightenment began in the 1600s and reached its height in the 1700s. Sometimes called the “age of reason,” this period saw a growth in skepticism about Christianity, the rise of Deism and atheism, the supremacy of reason over revelation, and a movement toward irreligion, particularly among the intellectual elites.

Or so we are told in our history classes.

While there is an element of truth in this description of intellectual life in the period, it ignores a number of important points. First, there were a number of counter-movements within the churches that kept Christianity vital for many, including among the elites. We see this in movements such as Jansenism among the Catholics, Pietism among the Lutherans, and Methodism in England.

Second, this view of the Enlightenment ignores the number of intellectual leaders who continued to hold to historic Christianity, such as Mersenne, Descartes, Pascal, Boyle, Newton, Hales …. Not all of these were completely orthodox in their views, but they were hardly Enlightenment rationalists when it came to religion. In fact, their religious faith helped impel them toward the kind of scientific advancement that is sometimes seen as the hallmark of Enlightenment thinking.

Perhaps the best example of an Enlightenment figure who made incredible contributions to the growth in knowledge of his era but who nonetheless maintained a robust Christian worldview is Leonhard Euler (pronounced roughly “oiler”). Although largely unknown outside of technical fields today, Euler was one of the greatest mathematical geniuses in history.

Leonhard was the son of Paul Euler, a pastor in the Reformed Church in Basel, Switzerland. Shortly after his birth, the family moved to Riehen, a town in the Canton of Basel, but Leonhard returned to the capital city when he began his formal education. He matriculated at the University of Basel at age 13, and completed his master’s degree in philosophy in 1723 at the age of 16. His dissertation compared the philosophies of Descartes and Newton.

He continued his studies in theology, Greek, and Hebrew in preparation for becoming a pastor, but the direction of his life changed radically due to the influence of Johan Bernouilli, the most famous mathematician in Europe and a family friend. Bernouilli was tutoring Leonhard in mathematics, and realized that the boy had a remarkable gift. Bernouilli convinced Paul Euler that Leonhard was destined to become a great mathematician, and so Lenhard changed the focus of his studies.

Euler completed a dissertation on the propagation of sound in 1726, and the following year took second place in a prestigious Paris Academy Prize Problem competition. (The subject was how to best place masts on a ship. He lost to Pierre Bouguer, who is known today as “the father of naval architecture.”) In future years, Euler would go on to win the prize twelve times.

Meanwhile, Bernouilli’s sons had taken positions at the St. Petersburg Academy of Sciences in Russia. Founded by Peter the Great (1672-1725), the academy’s purpose was to help Russia catch up to the West in education and the sciences. Many European scholars spent time working there due to its large endowment.

When one of the Bernouilli brothers died of appendicitis in 1726, the surviving brother recommended Euler for a position. When a professorship at the University of Basel failed to materialize, Euler went to Russia. He initially worked in the medical department and was a medic to the Russian navy, but he rose rapidly through the ranks at the academy. He became a professor of physics in 1731, and when Daniel Bernouilli left the academy in 1733, Euler became head of mathematics department.

In 1734, Euler married Katherina Gsell. They had 13 children, though only five survived their childhood. The following year, Euler came down with a fever that left him nearly blind in his right eye. He blamed the loss of vision on the work he had been doing on cartography, but his vision in that eye continued to deteriorate over the next decades.

In 1741, growing xenophobia in Russia led Euler to accept a position at the Berlin Academy under Frederick the Great. In the 25 years he spent there, he published over 380 articles, along with important books on mathematical functions and on differential calculus.

Frederick also asked Euler to mentor his niece, the Princess of Anhalt Dessau. Euler wrote her over 230 letters, which were later compiled into a bestselling book entitled *Letters of Euler on different Subjects in Natural Philosophy Addressed to a German Princess*. These letters deal not just with science and philosophy, but also address religious issues and reveal much of Euler’s personality and beliefs.

Unfortunately, Euler’s faith and his conservative, hardworking lifestyle did not sit well with the atmosphere at Frederick’s court, particularly after the arrival of Voltaire, the anti-Christian French satirist and *philosophe*. The situation in Russia had stabilized under Catherine the Great, so in 1761 Euler accepted an invitation to return to the St. Petersburg Academy of Sciences.

Five years later, Euler was diagnosed with a cataract in his left eye. Within a few weeks of its diagnosis, he was completely blind. This would normally have ended his career, but Euler became even more productive in some areas than he had been before. How could he do this?

- He had a photographic memory: he could recite Virgil’s
*Aeneid*verbatim, and could even tell his audience the first and last lines from any page of the edition he had learned. - His ability to concentrate was legendary: according to Condorcet, two of his students had summed seventeen terms in a complicated infinite series but could not agree on the fifteenth decimal point; Euler settled the argument by doing the calculation in his head and telling them the result.
- He was immune to distraction: he frequently wrote his treatises with his children playing at his feet.
- He had a prodigious capacity for hard work: in 1775, he produced roughly one mathematical treatise a week for the entire year.

By the end of his life, Euler had produced 886 papers and books filling roughly 90 volumes, making him the most productive mathematician in history with the possible exception of Paul Erdös (1913-1996). In fact, the last phase of his life was so prolific that the St. Petersburg Academy did not complete publication of his papers until 30 years after his death.

It is difficult for a non-mathematician to understand or describe Euler’s work. The best way to describe it is to simply note the range of his work. In mathematics, Euler developed many of the symbols for advanced calculations that are still in use today. He did work in infinitesimal calculus, geometry, trigonometry, algebra, graph theory, and applied mathematics. He also created the field of analytical number theory, uniting number theory and analytical mathematics for the first time.

He also worked in physics, astronomy, acoustics, and optics. He returned to the ancient liberal arts tradition and tried to reunite music and mathematics (with only limited success: his work was too mathematical for musicians and too musical for mathematicians). And he also introduced some significant advancements in logic.

All of this barely scratches the surface of Euler’s work. To put it in perspective, there is a Wikipedia entry entitled “List of Things Named after Leonhard Euler” that includes 78 items derived from his work, plus another nine named in honor of him. The entry notes that there is a joke among mathematicians and physicists that to avoid naming everything after him, discoveries and theorems are named after the first person *after* Euler to discover them.

Even with this prodigious output in mathematics and physics, Euler never left behind the theological commitments and interests of his youth. Like many Christian scholars of the time, he entered into the debates against the more anti-religious thinkers of his day. We see this in both his letters to Frederick the Great’s neice and in his *Rettung der Göttlichen Offenbahrung Gegen die Einwürfe der Freygeister* (*Defense of the Divine Revelation against the Objections of the Freethinkers*), a defense of biblical inspiration.

At times his mathematical work intersected with his philosophical and theological ideas. For example, following early Christian natural philosophers, he insisted that knowledge was based in part on precise quantitative laws, so that he dismissed philosophical systems that could not provide these laws as “heathen and atheistic.”

More often, however, his mathematical work was an expression of his deep faith as a Christian, who recognized that Jesus as the *logos* was the sum of all knowledge and truth. All his work was thus an expression of his worldview that recognized that every area of life was worth exploring as an act of worship and service of God.

**Dr. Glenn Sunshine is a professor of history at Central Connecticut State University and a Senior Fellow at the Colson Center for Christian Worldview.**