There were many prominent and gifted Enlightenment thinkers and authors. This is not the place to scrutinize them all; rather, I will briefly examine one representative from the beginning, one from the middle, and one from the end of this frantic time of philosophical thought and discourse.
(A BIRD’S EYE VIEW)
Leading Up to the Enlightenment
Within the Enlightenment
Emerging Out of the Enlightenment
THREE HISTORICAL CONTEXTS
THREE MONUMENTAL CHARACTERS
Figure 3 – Enlightenment/Three Monumental Characters
Rene Descartes – “I think therefore I am”
Our earliest Enlightenment writer is Rene Descartes, a French philosopher/ mathematician, who has the distinction of being called the father of modern philosophy. Chances are, if you have studied philosophy at a major university, you have read (or, at least, heard of) Descartes’ Meditations on First Philosophy.
Descartes is responsible for laying the foundation for rationalism, which can be defined as a theory of knowledge (epistemology) that “believes some ideas or concepts are independent of experience and that some truth is known by reason alone” (Hunnex, Chronological and Thematic Charts of Philosophies and Philosophers, 1986). Descartes’ works are considered the embryonic move of the era away from God’s revelation as the mainstay of man accesses to knowledge and toward man’s reason being the foremost means to that end. Needless to say, not all clergy have been overjoyed with Descartes and his work.
Archbishop William Temple once remarked that the most disastrous moment in European history was perhaps the bitterly cold day in the winter of 1619-1620 when French philosopher Rene Descartes began his journey toward rationalism...what Descartes did on that day began a trend that has not been reversed. (Dowley, 1995)
Descartes is especially well known for two items: His quote “I think therefore I am;” and his Cartesian method of inquiry. As to the quote, I have heard it quoted on no small number of occasions – although I suspect it is too often used by too many of us who have to little knowledge of what it actually means. What Descartes had in mind was
the mere fact I am thinking, regardless of whether or not what I am thinking is true or false, implies that there must be something engaged in that activity, namely an “I.” Hence, “I exist” is an indubitable and, therefore, absolutely certain belief that serves as an axiom from which other, absolutely certain truths can be deduced (Internet Encyclopedia of Philosophy, Rene Descartes).
Descartes’ Cartesian method of inquiry consists of four guidelines to move from opinion to science: a) Never accept anything as true unless it is clearly and inescapably so; b) Analyze or reduce a problem to resolvable parts; c) Organize particulars into general knowledge; and d) Check for completeness and negative cases (Hunnex, 41).
Although not a new concept, Descartes also set out to demonstrate in a quite systematic manner the concept of mind/body dualism. This was at the heart of his “I think therefore I am” quote. Not new to philosophy or religion, Descartes’ systematic analysis of this dualism was ready fuel for the dichotomy between faith and reason that was to gain great traction as this time period advanced.
Indeed, Descartes set the stage nicely regarding the place of rationalism and dualism for his like-minded, European mainland, enlightened brothers to follow.
David Hume “Reason is, and ought only to be, the slave of the passions”
David Hume sat on the opposite side of the fence and the opposite side of the English Channel from Descartes. Descartes was a French rationalist; Hume a British empiricist. As a dyed-in-the-wool empiricist, Hume believed that, “all ideas or concepts derive from experience and that truth must be established by reference to experience alone” (Hunnex). His famous quote, “Reason is, and ought only to be, the slave of the passions,” was no oblique assault on Descartes’ premise. Whereas Descartes zeroed in on reason, intellect, and our ability to think as the pathway to knowledge; Hume saw desire, emotion, and our ability to feel as the key.
In Hume’s world, ethics were based on a person’s feelings and not on some culturally derived, abstract moral principle; neither on some theologically contrived, absolute moral Creator. He writes, “Morals excite passions, and produce or prevent actions. Reason itself is utterly impotent in this particular. The rules of morality, therefore, are not conclusions of our reason” (Hume, Treatise, 325). And for Hume, it was the individual who sat as final judge regarding these “rules of morality.” His high view of free will granted individuals the right (and, indeed, the obligation) to act according to their desires and feelings with impunity and without input from the various social-cultural institutions such as religion and politics.
A number of other British thinkers of the era fell into the empiricism camp – among the better known are George Berkeley and John Locke. Locke became known as the father of modern liberalism. (Notice how so many of these men of the Enlightenment period ended up becoming known as the father of something – another testimony to the transhistorical influence of that era.) But it was Hume who was the primary harbinger in this realm of empiricism. He challenged the very basis of the (then) modern scientific methodology, arguing that just because something has always happened a certain way does not guarantee that it will always happen in that same manner. Thus epistemology and causality were turned on their heads. As was reason.
Immanuel Kant – “Thus, I had to deny knowledge in order to make room for faith”
By the time Immanuel Kant came along, the Enlightenment had been around long enough for most European intellectuals to realize they were in a bit of a pickle.
The Enlightenment commitment to the sovereignty of reason was tied to the expectation that it would...support certain key beliefs that tradition had always sanctioned. Crucially, these included belief in God, the soul, freedom, and the compatibility of science with morality and religion. Although a few intellectuals rejected some or all of these beliefs, the general spirit of the Enlightenment was not so radical. The Enlightenment was about replacing traditional authorities with the authority of individual human reason, but it was not about overturning traditional moral and religious beliefs.
The crisis (alias, pickle) was the fact “that the modern science and reason of the Enlightenment appeared to undermine the traditional moral and religious beliefs that free rational thought was expected to support.” It was an inevitable intellectual crisis. It was the conundrum of (on the one hand) replacing traditional authorities with the authority of individual human reason while (on the other hand) not overturning traditional moral and religious beliefs.
This is where Immanuel Kant enters the picture. Until quite late in his career Kant was a dyed-in-the-wool rationalist. Then he read Hume. And that got him to thinking (something Kant was not allergic to). With a newly formed, Hume-induced paradigm shift Kant set out to harmonize the opposing theories of rationalism and empiricism (Weeks, Philosophy in Minutes, p. 260).
Immanuel Kant’s Critique of Pure Reason was his response. He wrote it in 1781 – at the windup of the Enlightenment. In fact, it may well have been his writing that brought the era to a close. He wrote this particular volume after he had evolved considerably in his thinking regarding the concept of reason.
Critique of Pure Reason is not an easy read, to say the least. However, to this day, it remains one of the most widely known, studied, and influential works in modern philosophy. In this, his magnum opus, Kant’s goal was to determine where we “draw the line;” that is, where is the line between what we can know by way of reason and that which we can know only by way of experience.
With Descartes being a rationalist and Hume an empiricist, Kant, in attempting to bring these two concepts (and men) together, ended up an idealist – more specifically a transcendental idealist. In Kant’s model there are two worlds: 1) the phenomenon world, which is the world as we conceive of and experience it, the world as it appears to our minds; and 2) the noumenon world, which is the world as it really is (true reality). This former world being science and reason based. We can apprehend it; we can understand it; and we can test it. The latter world is transcendental. It is beyond us; beyond our experience, apprehension, and understanding. For those with a religious bent, Kant felt their belief in God, spirits, heaven, and hell may be beneficial simply because it helps such people cope with the challenges of life at a psychological level.
One thing for sure, when it comes to the Enlightenment, one certainly can’t count Kant’s critique inconsequential.
 Philosophers prior to Descartes who held to rationalism as a value would include Aristotle, Plato, and Aquinas. The latter attempted to bring together Greek rationalism and Christian revelation. Those who, in the ensuing Enlightenment era, followed Descartes in adopting rationalism include Baruch Spinoza, Gottfried Leibniz, and Immanuel Kant.
 This was the general pattern of Enlightenment thought in relation to geography – the rationalists, for the most part, being from the European mainland while the empiricists generally hailed from Britain. There are exceptions; for instance, Francis Bacon who developed the inductive method of reasoning and Isaac Newton (another Englishman) who formulated the laws of gravity and motion.
 Stanford Encyclopedia of Philosophy, Immanuel Kant, May 2010
 Kant’s initial Inaugural Dissertation stated that moral judgments are based on pure understanding alone. By the time he wrote The Critique of Pure Reason, he had modified that view substantially.
 In fact it was so difficult to read and comprehend that the early reviews of it were very few and uncomprehending (in Kant’s opinion) that Kant wrote a shorter and more accessible version entitled, Prolegomena to Any Future Metaphysics That Will Be Able to Come Forward as a Science (1783).
 Kant did attempt to condense and make his book more accessible by writing a somewhat simpler version, Prolegomena to Any Future Metaphysics, two years later.
 It must be noted that neither rationalism nor empiricism disregards the other school entirely. As Hunnex informs us, “the issue revolves on beliefs about 1) necessary knowledge and 2) empirical knowledge.” Wrapped up in that statement is the central issue of whether one can have genuine knowledge of the world without relying on experience.