René Descartes and John Locke, both seventeenth century philosophers, are often seen as two of the first early modern philosophers. Both Descartes and Locke attempt to find answers to the same questions in metaphysics and epistemology; among these: What is knowledge? Is there certainty in knowledge? What roles do the mind and body play in the acquisition of knowledge?
Descartes and Locke do not provide the same answers to these questions. In this paper I will consider the similarities and differences between the philosophies of Descartes and Locke. I will first briefly consider several similarities. I will then discuss a few important differences in their theories of knowledge (namely the distinction between rationalism and empiricism and the question of the existence of innate ideas). Finally, I will consider the more subtle distinction between the Cartesian and Lockean accounts of self, the role of the mind, and the mind/body distinction. Having enumerated similarities and differences, I will conclude that Descartes and Locke offer fundamentally different philosophies.
Although I maintain that the philosophies of Descartes and Locke are different, this does not exclude the possibility of similarities. In fact, I believe there are many points of agreement between Locke and Descartes. Locke's Essay Concerning Human Understanding is not a direct attack on Descartes; in contrast, it is an account of epistemology which, though not Cartesian, was influenced in part by Locke's reading of Descartes. Locke borrowed many of Descartes' philosophical ideas and objections and adopted much of his terminology.
I will now consider four passages in which Locke appears to be drawing on Descartes: the notions of ideas and qualities, the importance of language and reason, God and the will, and universals and classification. (Note that in these cases differences as well as similarities may be found, but I am here choosing only to address the similarities.)
Locke's notion of the idea is one example of a term borrowed from Descartes. For Locke, an idea is that which ``the mind perceives in itself, or is the immediate object of perception, thought, or understanding'' (Locke, 48). This seems to be exactly Descartes' definition of idea: ``whatever is immediately perceived by the mind'' (Descartes, 132). Locke then goes on to consider the qualities (powers to produce ideas) of external objects. He distinguishes between primary and secondary qualities; the latter are those which are not in the objects themselves but are perceived or sensed, while the former are those which cannot be separated from the object and belong to it at all times such as solidity, extension, figure, and mobility (Locke, 49). This echoes the distinction made by Descartes about the qualities of wax. Descartes clearly perceives (the having of) size, shape, and number, as well as motion, substance, and duration, but qualities such as color and sound (Locke's secondary qualities) are not as clearly perceived (Descartes, 92).
For both Descartes and Locke there is something about man which sets him apart from machines and animals. Descartes says that though machines may superficially appear to imitate man, they could still be discerned from real men with certainty. Machines, he says, ``could never use words, or put together other signs, as we do in order to declare our thoughts to others'' (Descartes, 44). Nor can animals (beasts), as they have not only less reason than men, but no reason at all (Descartes, 45). Compare this with Locke's claim that animals (brutes) ``have not the faculty of abstracting, or making general ideas, since they have no use of words'' (Locke, 64). In both Descartes and Locke, I see precursors for theories of the importance of reason and language.
Descartes and Locke both discuss free will; in particular, they consider how it is that our will may be both directed and remain free, and how it is consistent with the existence of a God that we can err in our ways. On the first of these points, Descartes thinks this does not limit our freedom; instead he believes that the more one inclines in one direction, the more free the choice is. On the second point Descartes believes that the imperfection is not one from God, but that our intellect is not as great as our will (Descartes, 101). Locke picks up on these sentiments, agreeing that for us as well as God ``to be determined by [one's] own judgment is no restraint to liberty'' (Locke, 106). Locke extends Descartes yet again by explaining that bad choices may arise from bad judgments (Locke, 110).
Finally, Locke adopts his ideas about universals and classification of genus and species from Descartes (Descartes, 179). Genera are used to group things according to similarity, while species are used to identify differences or differentia (Locke, 183). For both, genera and species are abstractions used for understanding.
I have focused thus far on ways in which Descartes and Locke are similar. Although I believe I have pointed to a number of undeniable similarities, I have not sketched anything resembling an overview of the philosophies of Descartes or Locke. I believe this is because their similarities are largely insignificant when compared with their differences. It is their differences, not similarities, which characterize their overall theories of knowledge.
For Descartes, knowledge depends on absolute certainty. Since perception is unreliable, indubitable knowledge cannot come from the outside world via the senses (Descartes, 76). Descartes believes that there are two ways of discovering knowledge: through experience and through deduction. If knowledge cannot come from experience of the outside world, then it must come from within. In contrast to perception, Descartes believes that deduction ``can never be performed wrongly by an intellect which is in the least degree rational'', so deductive knowledge is (the only) certain knowledge (Descartes, 2).
Such a system requires a basis of intuitively understood principles from which knowledge can be deduced. Descartes believes that there are some principles which are immediately known, such as the idea of the existence of the self and that of God's existence, These are principles which are ``revealed to [us] by natural light'' and ``cannot in any way be open to doubt'' (Descartes, 89). Descartes concludes that these principles are innate (Descartes, 97).
In contrast, Locke does not believe that there is any certain knowledge (Locke, 263). Instead, he believes in knowledge which is probable to a very high degree. Since he is not concerned with certainty, he need not abandon ideas based on perception. Knowledge can and does rely on the senses and observations. In fact, Locke says that all ideas come from sensation and reflection; all knowledge is founded on experience (Locke, 33).
Locke rejects the existence of any innate principles or ideas on at least two independent grounds. He argues that there are no innate ideas because, if there were, they would immediately be known to children, and they are not (Locke, 8). Locke also explains that if any idea is innate, the idea of God is innate. However, since there is not a universally agreed upon notion of God, the idea of God cannot be innate (Locke, 25-26). There are no innate ideas present in the mind; instead, the mind is a ``white paper, void of all characters, without any ideas'' (Locke, 33). It is through experience, not some divine natural light, that the mind can gain knowledge.
In summary: Both Descartes and Locke are initially skeptics about the possibility of certain knowledge. Descartes is a continental (French) rationalist, believing that there is certain knowledge and that human reason (innate ideas and deductions thence) is the sole source of such knowledge. Locke, in contrast, is an English empiricist who believes that knowledge is not certain, but that extremely probable knowledge can be gathered from experience. The idea of (certain enough) knowledge arising from experience is inconceivable to Descartes, just as the existence of innate ideas in the mind is unacceptable to Locke. The philosophies of Descartes and Locke diverge irreconcilably on the question of the origin of knowledge.
Both Descartes and Locke try to explain what the self is and how the mind and body are linked. It is clear what Descartes thinks he (the self) is. In his Second Meditation, he asserts that he is a ``thinking thing'' (Descartes, 82), a thing that thinks: ``doubts, understands, affirms, denies, is willing, is unwilling, and also imagines and has sensory perceptions'' (Descartes, 83). Not only does Descartes consider the self to be a thinking thing, but he believes that is his essence (Descartes, 114).
Descartes makes an important distinction between the mind or thinking substance (res cogitans) and the body or extended substance (res extensa). He believes there is a link between the soul (mind) and body through which sensations (in particular, pain) are transferred and that this link allows one to identify a body as one's own. ``I am not merely present in my body as a sailor is present in a ship, but that I am very closely joined, and, as it were, intermingled with it, so that I and the body form a unit'' (Descartes, 116). But he maintains that ``it is certain that I am really distinct from my body, and can exist without it'' (Descartes, 115). For Descartes, then, `self' refers to the soul or the mind alone, not the body.
Like Descartes, Locke makes a distinction between the body, ``an extended solid substance, [and the soul,] ``an immaterial spirit ... a substance that thinks'' (Locke, 124). Locke thinks the soul and body are separate, but related. Also like Descartes, he thinks of the self as a thing that thinks. But he does not agree with Descartes that his ``essence consists solely in the fact that [he is] a thinking thing'' (Descartes, 114). Instead, Locke thinks that the self is both the mind and its body, not the ``thinking or rational being alone'' (Locke, 138). Unlike Descartes who considers thought at a given moment, Locke goes on to give an account of memory and explains identity (sameness of self) in terms of continuity of consciousness (Locke, 139).
The Cartesian and Lockean theories of identity, then, can be explained as follows: Both distinguish the mind or soul from the body, and both explain personal identity in terms of thinking (although only Locke considers persistent thought). Descartes says that the self is, and is only, the thinking soul. In contrast, Locke says the self is both the soul and the body. Locke's account of identity seems not to be directly in conflict with Descartes' (as is the case for their theories of innate ideas). It is worth noting that in many ways Locke's account of identity appears to be a superset of that offered by Descartes, rather than a completely separate theory.
In this paper I have explored some similarities and differences in the philosophies of Descartes and Locke. The similarities I have considered seem to me weak when compared with the differences. However, I believe that it is not entirely accurate to say that their philosophies are different and leave it at that. Both philosophers are abandoning older traditions and offering new ways of looking at knowledge and skepticism. In many ways Locke is drawing on Descartes, rejecting some of his ideas, accepting some, and extending others. Nonetheless, I have tried to illustrate that despite a historical connection and many similarities, their philosophies are fundamentally different. In particular, Descartes and Locke disagree on the origins of knowledge, innate ideas, and the meaning of the self. They offer thoroughly different answers to these and other important philosophical questions.