Descartes is among my least favorite of those conventionally termed great philosophers. My treatment to date has been mainly limited to a few dismissive remarks. Here I’d like to add a few “historiographical” points of demarcation.
Insofar as there is general consensus among scholars, Descartes (1598-1650) first and foremost has claim to fame as a very influential promoter of something recognizably close to modern scientific method. He is often credited with the invention of analytic geometry, based on an early recognition of the systematic isomorphism between geometry and algebra. Galileo (1564-1642) had already taken up an approach to natural science based on mathematical analysis, which Descartes enthusiastically adopted. Descartes particularly promoted a methodology based on clear and distinct ideas, which he held to give certain knowledge. He advocated an orderly progression from the simple to the complex.
On a broad social level, Descartes is remembered for promoting the independence of scientific investigation, particularly from the doctrinal concerns of the Catholic Church. But he was also a religious thinker. While confessing in a private letter that he did not literally believe various details of received scripture, he was very engaged with proofs of the existence of God.
Numerous scholars have pointed out that outside the domains of mathematics and natural science, Descartes in many ways remained close to the Latin scholasticism of which he has been commonly regarded as the slayer. In Descartes and the Modern Mind (1952), for example, Albert Balz argued at length that the thought of Thomas Aquinas was an essential precursor to Descartes. I note that Augustine had already emphasized the importance of the interpretive role of reason, even in matters of faith. On Aquinas’ account, God gives us not only revelation, but also the natural light of reason. In Summa Contra Gentiles, Aquinas addressed questions of philosophy and theology entirely from the point of view of that “natural light”, while aiming to show that the natural light of reason independently leads to many of same truths he attributed to revelation. Descartes makes great use of a similar concept of the natural light of reason.
Both Descartes and Aquinas thought the natural light of reason, properly understood, gives us truths, full stop. I think it gives us invaluable criteria for judgment and interpretation, while always in principle leaving room for discussion about what conclusions should be drawn. I also think the “natural light” itself comes to us in degrees, and is never a simple or unproblematic possession.
A different strand of Latin scholastic thought tended to claim that all human knowledge originates in sensible images, while attributing such a view to Aristotle. (I think this is overly strong, and that Aristotle only meant to defend the pragmatic value of sensation against Platonic skepticism about all deliverances of sense.) Descartes famously argued that sensible images are not the only source of knowledge, and I think that is true, as far as it goes.
Here is where Descartes’ theses about clear and distinct ideas come into play. A methodological discipline based on examining whether our ideas are clear and distinct is an important source of human knowledge. Again, this much I can agree with, but I think clarity and distinctness are relative criteria and not absolutes. As relative criteria, they have been implicitly employed by most if not all serious thinkers. I take such evaluations to have been a major implicit concern of Platonic and Aristotelian dialectic, which in part aims to discern meanings that are more clear and distinct.
Descartes effectively claimed that clarity and distinctness are absolute, decidable properties of ideas. One of the broadly scholastic views he sharply criticized was that our best knowledge of sensible things is only “probable”. Descartes claimed that the results of his methodological analyses were certain, in the same way that mathematical conclusions follow with certainty from their premises. This goes well beyond the claim that there is practical value in such methodology.
Building on arguments of Augustine and Avicenna, he also famously gave great importance to the claim that immediate contents of the mind give evidence of unconditional certainty of the abstract existence of something. The very possibility that I could be deceived implies the abstract existence of an abstract something that could be deceived. Further, if something in any way appears to me to be such and such — even if I am wrong about all the details — independent of all the details, the barest fact of the appearance implies that some appearance generically “exists”.
The “I” that is in this way proven to exist and the appearance that is proven to exist are both extreme abstractions. Even Descartes did not claim that either of these existences by itself gives us any further knowledge. From this basis alone, I could still be entirely mistaken about the kind of being that I am, and about every detail of what appears to me to be the case. In spite of the famous Cartesian doubt, Descartes actually wanted to makes strong claims of certainty and to refute skepticism. Many readers have concluded, though, that he opened the door for a new, more global form of skepticism, because what he clearly establishes as certain seems so utterly lacking in content.
I would hasten to add that unreasonable, excessive skepticism about human knowledge is best refuted by successful achievements of goals in real-life situations. Only a hypocrite could claim to live in the world with no well-founded beliefs whatsoever. The ancient Skeptics were only “skeptical” about theoretical accounts of things, not about practical concerns of everyday life.
By setting the bar too high and aiming at absolute certainty, Descartes actually opened the door for more radically subjectivist views that no one in the ancient world would have taken seriously (and not just because ancient people were naive). At the same time, he was very impatient with “dialectic”, and tended to foreshorten discussions of meaning and interpretation, in favor of claims that certain contents are unequivocally clear and distinct. Thus the ultimate result of his thought oscillates unstably between extremes of “Cartesian” skepticism and dogmatism.
Another point on which Descartes has been very influential is his strong representationalism. For Descartes, strictly speaking we never have practical knowledge of things in the world, only knowledge about contents of our mental representations, insofar as they are clear and distinct. In particular, we only know bodies through our mental representations of them. Rather than consisting in an interpretive stance of a situated being in the world, the Cartesian cogito achieves its purely subjective certainty in a way that is supposed to be peculiarly “outside” the real world altogether.
Unlike the representationalism of Locke, which is grounded in a kind of empirical psychology, that of Descartes is closely bound up with an ontological mind/world dualism more radical than anything Plotinus, Augustine, or Avicenna ever contemplated. For Plotinus, Augustine, and Avicenna, the soul was a very special kind of “something” existing in the real world, even if for Plotinus and Augustine it was not a “subject” in the sense of something underlying something else. For Locke — the other great early modern promoter of representationalism — our mental worlds are ultimately contained within the natural world. For Descartes, there is the world and there is the soul, and never the twain shall meet. The soul has its own mental world where it seems to relate directly only to God, and human knowledge occurs only in that mental world.
It is due to this unprecedentedly radical mind/world dualism of Descartes, I think, that virtually no one — even among his admirers — wants to uphold his metaphysics. This is an extreme example of what Hegel called “alienation”.