The Enlightenment was a time to exalt autonomous human reason, spur reliance on the past and superstition, and depend on oneself for knowledge. Among the topics challenged was the spiritual realm of thought. The ideas of John Locke, David Hume, and Immanuel Kant shifted how people treated truth, morality, miracles, and beauty.

John Locke (1632-1704)

John Locke
Portrait by Godrey Kneller (1697)

In America, John Locke is most well-known for influencing Thomas Jefferson’s writing of the Declaration of Independence. The document from which the Declaration owes some of its principles, An Essay Concerning Human Understanding, also proposed another major element of Locke’s thinking: the theory of tabula rasa.

Tabula rasa, a Latin phrase that means “clean slate” or “blank slate”, suggested that at birth the human mind had no data or rules for its processing. He rejected the “popular view that people have native ideas, original beliefs, stamped upon their minds at their creation. Rather, he upheld that through experience – sense-based reason – ideas come to inhabit the mind. This sense-based experience perspective coalesces more with Bacon, who similarly believed in the power of the senses in discovering knowledge, than Descartes.

From a Christian viewpoint, the tabula rasa theory removes man’s innate knowledge of natural law (Ro. 2:14) and sin consciousness (Ro. 3:20).

David Hume (1711-1776)

David Hume
Portait by Allan Ramsey (1754)

David Hume is the picture of skepticism. When he died in 1776, he made clear to those who last saw him that he would die as he lived – devoid of religious sentiment.

Like Locke, Hume pursued knowledge from an empiricist standpoint, but he differed in in his recognition of the impossibility of certainty. Nevertheless, experience yielded more sure results than reason alone, as a rationalist like Descartes may assert, as only by experience can one conclude anything about causality. Hume held to what A.J. Ayers in the 20th century would call the Verification Principle. According to this principle, knowledge is only that which is scientifically verifiable. This, of course, excludes the immaterial such as miracles.

In his 1768 publication Essays and Treatises on Several Subjects, Hume attacked the concept of miracles in the aptly-named article “Of Miracles.” Against the existence of miracles, he argued that 1) no large and credible audience had witnessed any alleged miracle, 2) the proclivity of human nature to the spectacular makes man disposed to believe to be a miracle that which is not, and 3) the belief in miracles finds particular prevalence among barbaric, unenlightened peoples.

Photo from Premier Christian Radio

The second proof against miracles ties to Hume’s criticism of religion. Hume claimed that man invented religion to answer questions about supposedly unknown causes of life and death, plenty and want, et cetera. The learned man knows that the causes are not so unknown, but rather stem from the “particular fabric and structure of the minute parts of their own bodies and of external objects” (Natural History of Religion, sec. III). For the multitude, such a plain answer does not satisfy the affinity for the spectacular. They create instead a deity, who oftentimes takes the form, nature, and qualities of man – basically, a super-powered human.

Having rejected God and having gone the empirical route, Hume needed a basis for morality and social ethics apart from a Transcendent Reality or reason. His answer: sentiment and emotion. As he wrote in An Enquiry into the Source of Morals, moral action “depends on some internal sense or feeling that nature has made universal in the whole species” (An Enquiry, sec. I).

Immanuel Kant (1724-1804)

Johann Gottlieb Becker
Portrait by Johann Gottlieb Becker (1768)

Immanuel Kant was raised in a pietist environment, which emphasized religious affections and downplayed doctrine. While he maintained Christian principles through his life, he essentially left the church because he struggled to reconcile faith with science.

While Descartes took a hard rationalist stance and Bacon, Locke, and Hume leaned more empiricist, Kant recognized the merits of both. In Critique of Pure Reason, Kant proposed a theory called transcendental idealism. In this view, the mind imposes structure on sensory data; objects and events correspond or accommodate to the working of the human mind, which is a filter of time and space. We do not perceive objects as they really are, but rather through the filter that the mind provides, so they have “appearances” that we can perceive and study.

noumenal phenomenal divide
Artist unknown

Kant called the world in which objects exist as they really are noumenal To use Platonic language, this is the world outside the Cave. The altered world that we create through the mental processing of sense perception is the phenomenal – the material and scientific. Because the noumenal exists outside the time and space by which the actual is rearranged into the perceivable, knowing the noumenal is impossible. To attain any knowledge of the noumenal requires that God break the barrier between it and the phenomenal.

Later thinkers would indirectly provide alternate interpretations of the noumenal-phenomenal divide.

Of morality: Hume concluded that moral goodness was a product of emotion. In Critique of Practical Reason, Kant credited categorical imperatives. The categorical imperatives, basically revisions of the Golden Rule, had that men act by the maxim that can become universal law and act as you’d expect all else to act. In other words, moral action is that which you believe or know the vast majority would consider right. (Finding problems with categorical imperatives is not hard.)


Kant called Enlightenment “man’s emergence from his self-imposed nonage [inability to reason for oneself]” (“What Is Enlightenment?”, par. 1). Philosophers probed the depths of questions that had plagued men since ancient Greece and of questions that hadn’t concerned men until those centuries that religion fell under scrutiny. Later intellectual movements like existentialism formed in response to these Enlightenment ideas.

For the text of Locke’s An Essay Concerning Human Understanding, see this translation.

For an overview of the philosophies of Francis Bacon and Rene Descartes, see this post.