15 The Rationalists

The Rationalists

Is all of our knowledge based on the evidence of the senses, or is some of it justified by other means? This epistemological question about the foundations of knowledge is what separates Rationalism and Empiricism. According to Rationalism at least some knowledge can be had through reason alone. For rationalists, the paradigm example of knowledge acquired independent of sense experience is mathematics. Once we have the concepts required to understand mathematical propositions (like 2+2=4), no experience is required to be justified in accepting their truth. They seem to be adequately known “through the light of reason.” Empiricism, on the other hand, takes all of our knowledge to be ultimately grounded in sense experience. Descartes was the first significant rationalist philosopher of the modern classical period. He rejects sense experience as a trustworthy source of knowledge early in his Meditations. Following Descartes, a number of other European philosophers develop rationalist philosophical systems. Leibniz and Spinoza are the most notable. Meanwhile, an empiricist tradition gets started in Great Britain.

The three major empiricist philosophers are John Locke, Berkeley and David Hume. In this chapter we will focus on Descartes, Spinoza, and Liebniz, and we will take up empiricism in the next chapter.


Rene Descartes (1596 – 1650) lived during an intellectually vibrant time. European scholars had supplemented Catholic doctrine with a tradition of Aristotle scholarship, and early scientists like Galileo and Copernicus had challenged the orthodox views of the Scholastics. Surrounded by conflicting yet seemingly authoritative views on many issues, Descartes wants to find a firm foundation on which certain knowledge can be built and doubts can be put to rest. So he proposes to question any belief he has that could possibly turn out to be false and then to methodically reason from the remaining certain foundation of beliefs with the hope of reconstructing a secure structure of knowledge where the truth of each belief is ultimately guaranteed by careful inferences from his foundation of certain beliefs.

When faith and dogma dominate the intellectual scene, “How do we know?” is something of a forbidden question. Descartes dared to ask this question while the influence of Catholic faith was still quite strong. He was apparently a sincere Catholic believer, and he thought his reason-based philosophy supported the main tenants of Catholicism. Still he roused the suspicion of religious leaders by granting reason authority in the justification of our beliefs.

Descartes is considered by many to be the founder of modern philosophy. He was also an important mathematician and he made significant contributions to the science of optics. You might have heard of Cartesian coordinates. Thank Descartes. Very few contemporary philosophers hold the philosophical views Descartes held. His significance lays in the way he broke with prior tradition and the questions he raised in doing so. Descartes frames some of the

big issues philosophers continue to work on today. Notable among these are the foundations of knowledge, the nature of mind, and the question of free will. We’ll look briefly at these three areas of influence before taking up a closer examination of Descartes’ philosophy through his Meditations of First Philosophy.

To ask “How do we know?” is to ask for reasons that justify our belief in the things we think we know. Descartes’ Meditations provide a classic example of the epistemological project of providing systematic justification for the things we take ourselves to know, and this remains a central endeavor in epistemology. This project carries with it the significant risk of finding that we lack justification for things we think we know. This is the problem of skepticism. Skepticism is the view that we can’t know. Skepticism comes in many forms depending on just what we doubt we can know. While Descartes hoped to provide solid justification for many of his beliefs, his project of providing a rational reconstruction of knowledge fails at a key point early on. The unintended result of his epistemological project is known as the problem of Cartesian skepticism. We will explain this problem a bit later in this chapter.

Another area where Descartes has been influential is in the philosophy of mind. Descartes defends a metaphysical view known as dualism that remains popular among many religious believers. According to this view, the world is made up of two fundamentally different kinds of substance, matter and spirit (or mind). Material stuff occupies space and time and is subject to strictly deterministic laws of nature. But spiritual things, minds, are immaterial, exist eternally, and have free will. If dualism reminds you of Plato’s theory of the Forms, this would not be accidental. Descartes thinks his rationalist philosophy validates Catholic doctrine and this in turn was highly influenced by Plato through St. Augustine.

The intractable problem for Descartes’ dualism is that if mind and matter are so different in nature, then it is hard to see how they could interact at all. And yet when I look out the window, an image of trees and sky affects my mind. When I will to go for a walk, my material body does so under the influence of my mind. This problem of mind-body interaction was famously and forcefully raised by one of the all too rare female philosophers of the time, princess Elisabeth of Bohemia.

A whole branch of philosophy, the philosophy of mind, is launched in the wake of problems for substance dualism. Today, the philosophy of mind is merging with neuroscience, cognitive psychology, and information science to create a new science of mind. We are rapidly learning how material brains realize the processes of thought. Once again, Descartes has failed in a most fruitful way. We also see how undeserved philosophy’s reputation for failing to answer its questions is. While many distinctively philosophical issues concerning the mind remain, the credit for progress will go largely to the newly minted science of mind. The history of philosophy nicely illustrates how parenthood can be such worthwhile but thankless work. As soon as you produce something of real value, it takes credit for itself. Later in a chapter on the

philosophy of mind we will examine some developments in this area since Descartes and get acquainted with a few of its contemporary issues including the nature of consciousness.

The final big issue that Descartes brought enduring attention to is the problem of free will. We all have the subjective sense that when we choose something we have acted freely or autonomously. We think that we made a choice and we could have made a different choice. The matter was entirely up to us and independent of outside considerations. Advertisers count on us taking complete credit and responsibility for our choices even as they very effectively go about influencing our choices. Is this freedom we have a subjective sense of genuine or illusory? How could we live in a world of causes and effects and yet will and act independent of these? And what are the ramifications for personal responsibility? This is difficult nest of problems that continues to interest contemporary philosophers.

Descartes’ is also a scientific revolution figure. He flourished after Galileo and Copernicus and just a generation before Newton. The idea of the physical world operating like a clockwork mechanism according to strict physical laws is coming into vogue. Determinism is the view that all physical events are fully determined by prior causal factors in accordance with strict mechanistic natural laws. Part of Descartes’ motivation for taking mind and matter to be fundamentally different substances is to grant the pervasive presence of causation in the material realm while preserving a place for free will in the realm of mind or spirit. This compromise ultimately doesn’t work out so well. If every event in the material realm is causally determined by prior events and the laws of nature, this would include the motions of our physical bodies. But if these are causally determined, then there doesn’t appear to be any entering wedge for our mental free will to have any influence over out bodily movements.

Now we will turn to Descartes’ Meditations and examine how he comes to the positions just outlined. Here is a link to several of Descartes’ writings including Meditations on First Philosophy: http://www.earlymoderntexts.com/authors/descartes

The Meditations

Descartes project in his meditations is to carry out a rational reconstruction of knowledge. Descartes is living during an intellectually vibrant time and he is troubled by the lack of certainty. With the Protestant Reformation challenging the doctrines of the Catholic Church, and scientific thinkers like Galileo and Copernicus applying the empirical methods Aristotle recommends to the end of challenging the scientific views handed down from Aristotle, the credibility of authority was challenged on multiple fronts. So Descartes sets out to determine what can be known with certainty without relying on any authority, and then to see what knowledge can be securely justified based on that foundation.

In the first meditation we are introduced to Descartes’ method of doubt. According to this method, Descartes goes through all of his beliefs, not individually but by categories, and asks whether there is any possible way that beliefs of this or that type can be mistaken. If so, they

must be set aside as doubtable. Many of these beliefs may ultimately be redeemed as knowledge, but they cannot serve as part of the secure foundation of indubitable beliefs from which his rational reconstruction of knowledge proceeds. Empirical beliefs, things that we believe based on the evidence of our senses, are set aside first. Our senses sometimes deceive us, as when an oar appears bent in water or a stranger in a crowd appears to be a friend. It won’t do to say that we can reliably diagnose these cases and correct for mistaken appearances though because we also have experiences just like seemingly reliable sense experiences that are anything but in the case of dreams. How can we be certain that any of our seeming sense experiences of the external world aren’t in fact dreams? How can we be certain that our whole life isn’t a dream?

So sense experience is set to the side as uncertain and insufficient for justifying knowledge. Descartes then considers things we might know for certain by the light of reason, like mathematical claims. I seem to be about as certain in my belief that 2+2=4 as I can be about anything. Is there any possible way I could be mistaken? Descartes here imagines a powerful demon that could deceive me into always thinking that 2+2=4 when in fact this is not true. Is this a genuine possibility? Descartes allows that it is and considers all such knowledge had through reason doubtable as well.

Does anything remain? Are there any beliefs that can’t be doubted, even given the hypothesis of a powerful evil deceiver? Descartes does find at least one. Even an evil deceiver could not deceive Descartes about his belief that he thinks. At least this belief is completely immune from doubt, because Descartes would have to be thinking in order for the evil deceiver to deceive him. In fact there is a larger class of beliefs about the content of one’s own mind that can be defended as indubitable even in the face of the evil deceiver hypothesis. When I look at the grey wall behind my desk I form a belief about the external world; that I am facing a grey wall. I might be wrong about this. I might be dreaming or deceived by an evil deceiver. But I also form another belief about the content of my experience. I form the belief that I am having a visual experience of greyness. This belief about the content of my sense experience may yet be indubitable. For how could the evil deceiver trick me into thinking that I am having such an experience without in fact giving me that experience? So perhaps we can identify a broader class of beliefs that are genuinely indubitable. These are our beliefs about the contents of our own mind. We couldn’t be wrong about these because we have immediate access to them and not even an evil deceiver could misdirect us.

The problem Descartes faces at this point is how to justify his beliefs about the external world based on the very narrow foundation of his indubitable beliefs about the contents of his own mind. And this brings us to one of the more famous arguments in philosophy: Descartes’ “Cogito Ergo Sum” or “I think, therefore I exist.” Descartes argues that if he knows with certainty that he thinks, then he can know with certainty that he exists as a thinking being. Many philosophers since then have worried about the validity of this inference. Perhaps all we are entitled to infer is that there is thinking going on and we move beyond our indubitable foundation when we attribute that thinking to an existing subject (the “I” in “I exist”). There are issues to explore here. But bigger problems await Descartes, so we will just note this one and let it pass.


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A Brief Introduction to Philosophy by Southern Alberta Institution of Technology (SAIT) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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