Descartes believed that the physical world can be explained in terms of mechanical laws that govern the ways in which matter is in motion.
At the same time, Descartes was also a devout Christian, committed to the various church doctrines about an immaterial and immortal soul.
The Discourse of Method gives a brief intellectual autobiography. The Meditations seek to explain how minds, or the soul, fits into the mechanistic picture of the physical world.
We made special note of Descartes’s remarks about the difference between human and non-human animals in the Discourse.
Suppose there was a non-human animal or machine that acted very much like a person. How would we know it isn’t one?
Descartes’s answer involves language. Only human beings are capable of rearranging the words they use to say an indefinite variety of things. Other animals, or machines, can vocalize and even approximate the sound of human speech. But they can’t express the same variety of thoughts, or respond to the same variety of questions, as human beings can.
He concludes that the behavior of animals is purely mechanical, like clocks (Descartes [1637] 2019, 33).
When we read Hobbes, a materialist, philosopher, we will see that he makes at least superficially similar observations about language while drawing less severe conclusions about the minds of non-human animals. Then again, Hobbes thought that even human minds were like clocks, so make of that what you will.
The project is to provide a foundation for everything he believes. In order to do that, he has to “raze everything to the ground and begin again from the original foundation” (Descartes [1641] 2019, 40).
How? The method of doubt. He is going to withhold his assent “from opinions that are not completely certain and indubitable,” treating them the same as those that are “patently false” (Descartes [1641] 2019, 41).
More specifically, he’s going to take aim at a class of beliefs that are based, ultimately, on the senses. He’s going to come up with a reason to doubt them all, thus clearing the decks quickly.
Two arguments for general doubt:
In both cases, he says, there is a possible explanation for why I have all of the sensations that I, in fact have. But if that explanation is correct, everything I think I learn from the senses is false. Rather than being caused by a tree, my visual sensation that looks like a tree is caused by a dream or a demon. There might be no tree there at all.
The reason that these hypotheses are a source of doubt is that he cannot use the senses to say whether he is in the real world, inhabited by all the things that he believes he sees, hears, smells, touches, and tastes, or a dream world that might include none of those things.
Given that, how can he know anything? Reconstructing his knowledge is the project of the rest of the meditations.