A. C. Grayling's biography of Descartes helped me see how Descartes used doubt: he was a "methodological sceptic," not a "problematic sceptic" (282); doubt for Descartes was a tool for argument and not a fundamental problem. Grayling states this quite fully:

The sceptical arguments employed in the method of doubt do not themselves have to be plausible or sustainable. They may indeed be far less plausible than what they impugn; but that does not matter. They are simply a device, an heuristic, something that helps one to see how it is that when one says "I exist" it cannot but be true. (284)

Grayling also points out that even the Cogito itself is not entirely original with Descartes:

When St. Augustine wrote in the early fifth century AD that we can doubt everything except the soul's doubting ("On Free Will" II 3:7) he was even then not inventing a new idea, and presumably Jean de Silhon, who published his The Two Truths in 1626 containing the remark "it is not possible that a man who has the ability, which many share, to look within himself and judge that he exists, can be deceived in this judgment, and not exist", must have known St. Augustine's remark or — independently of that — the idea itself in the philosophical tradition. (278-279).

Grayling spends a great deal of time discussing Galileo and his conflict with the Inquisition, because Galileo's punishment had a significant influence on Descartes: it kept him from publishing Le Monde and hence, in the long run, played a great role in the forms his work took. Had Descartes published Le Monde, Grayling's book implies, then he would not have written The Discourse on Method and The Meditations! Galileo's punishment made Descartes back down out of fear of similar sanctions—and, Grayling emphasizes, Descartes backed down because he was a devout Catholic who did not want to run afoul of the Church.

Grayling's discussion of Galileo, then, taught me something that I had not registered previously (neither when reading Galileo in SLE nor when studying Brecht's Leben des Galilei in graduate school):

Galileo published his telescopic discoveries in a little book called Sidereus Nuncius (the Starry Messenger). But in the next couple of years he made more accurate observations of the moons of Jupiter and, while puzzling over inconsistencies in his data, realized he had to take into account variables in his own position relative to the motions he was observing—specifically, variables which had to be caused by the motion of earth round the sun, thus showing that Copernicus's model was not, as Copernicus himself had taken it, merely a convenience for simplifying calculations of the motions of heavenly bodies, but actually correct. (166)

The new point to me is that Galileo did not just believe that Copernicus's model was an accurate description of the universe; instead, he demonstrated that it is. (For a fascinating book on the Catholic Church's ambivalent relationship to astronomy, I recommend J. L. Heilbron's 1999 study The Sun in the Church: Cathedrals as Solar Observatories. The Church needed astronomers to calculate the correct dates for Easter, but feared the implications of their research.)

Finally, Grayling also nicely summarizes contemporary theories of consciousness (all of which dispense with Descartes's mind-body dualism):

Consciousness has arisen amongst higher mammals, according to these theories, because of its survival advantage—an organism’s appropriate use of energy and protection from harm are much enhanced when it is able to place itself in a map of the environment and make plans about the best courses of action in it. Creatures which are merely biological automata, even if highly sensitive to their surroundings, would not be as adaptive as creatures that are genuinely conscious. (288)

This succinctly captures the role of evolution in the development of consciousness, while also emphasizing, pace Descartes, that animals are also conscious beings (and not mindless machines). Or rather, Grayling points out how human beings are also animals and are as subject to evolution as any other living species is.