(Remember that Descartes had to be careful to avoid the kind of problems that Galileo
encountered when Galileo came up with research techniques that were new: Galileo was accused
essentially of impiety, and was convicted. Descartes had to find a way to show not only that his
method is effective in practice, but that its procedures and results do not involve anything that
violates religious principles. This he addressed in Parts Three and Four.)
A. Therefore in Part Four, Descartes must do 3 things:
1. Demonstrate the use of the method in philosophy (the basis of the natural sciences, he says), showing what he has discovered by using the method;
2. Show that using the method properly really leads to the discovery of true things, not false or doubtful things (that is, show that what he is learning by using the method is really knowledge);
3. Show that using the method does not involve violating any laws or religious principles.
B. Here's how he does those three things (or, here's how he says he does them):
1. According to Descartes, he was able to use the method first of all to discover 2 basic true things:
a. He realized that every time he asked himself what was true, and every time he doubted whether his old opinions were true, he was thinking. So he could not deny that he was thinking, at those times. And if he was thinking, he realized, he must be existing too. So he concluded that the statement 'I think, therefore I am' must be true every time he thought or said it.
Be careful: Descartes does NOT say that the statement 'I am, therefore I think' must be true. He is certainly aware that there could be things that exist but do not think, as he shows in Part Five. The things that exist but do not think are of course not aware of their existence while they are not thinking.
b. He tried to pretend he had no body and no material aspect, and says that even while doing this he could not pretend he did not exist: in order to pretend, one must think, and if one thinks, one must exist. Therefore he concluded that he "was a substance the whole essence or nature of which was simply to think, and which, in order to exist, has need of no place nor depends on any material thing" (page 19): in other words, he concludes that what he really is is a mind or soul or spirit, and that mind is independent of body. (We just happen to have bodies now because the divine being gave them to us; but we could exist without them, he thinks.)
Note: This second conclusion seems to depend on the assumption that if one is not conscious of
any influence of body on mind, or if one is not aware of any way in which mind could depend on
body for its existence, then mind must not depend on body. This in turn assumes that if one is not
aware of something and has no evidence of that something, then that something does not exist.
These assumptions are not necessarily true, and at any rate deserve investigation.
Descartes puts these two conclusions to one side for the rest of Part Four, but he will do more
with them in Parts Five and Six.
2. Now Descartes attempts to show why his conclusions should be accepted as true. This means he will try to provide a foundation for knowledge. That is, he must try to prove that what he has learned with the method is in fact knowledge.
a. First he considers what makes these conclusions special among his ideas: what gives them their certainty; why do they seem so surely true? They seem to him to be totally clear and undeniable. He judges that all of his "clear and distinct" ideas must be true, though he realizes that there may be some difficulty in determining which ones are really totally distinct (that is, it may happen that a person could fail to see where one idea leaves off and the next begins, and may then confuse two things).
But, what if anything guarantees that our clear and distinct ideas are true? After all, humans make mistakes all the time. Could we not be mistaken when we think that our clear and distinct ideas are true? Can we tell what is true and real at all?
Descartes thinks that in fact something does guarantee that our clear and distinct ideas are true.
Because of this guarantee, we can tell what is real and true at least some of the time. Therefore
according to Descartes real knowledge is indeed possible in science, mathematics, philosophy,
and other fields.
b. Descartes thinks that only a perfect divine being could guarantee that our clear and distinct ideas are true, and he sets out to show that a perfect divine being exists. (Note: When Descartes uses the term 'perfect' he means 'without limitations or lacks.' The English word 'perfect' is being used to translate a Latin word that means 'complete.' This Latin word had been used in medieval Christian theology to describe God, and Descartes is continuing this usage. He wants to meet his potential critics in the Church on their own terms, and show that their conceptions actually support his own ideas.)
Now, why would we need a perfect divine being to guarantee that our clear and distinct ideas are true? Certainly we could not ourselves guarantee that any of our ideas are true: we make mistakes and don't yet know about many things; and we do not have the power to make the facts fit our ideas.
But something that had all the "perfections" of the Judeo-Christian-Islamic divine being(1) would be able to guarantee that our clear and distinct ideas are true. These "perfections" are listed on page 20: this divine being is infinite (unlimited, in space, time, power, etc.), unchanging, all-knowing, and all-powerful. (On page 22 Descartes also says that this divine being is all-truthful and benevolent, so that it would not deceive us systematically.)
Why would these particular qualities be required in order to guarantee that our clear and distinct ideas are true? Look at it this way: If there was no being that was all-powerful, then there would be no being that was strong enough to make the universe be the way this being wished it to be. The universe could change in a way that was beyond this being's control, and the being could not guarantee anything. But an all-powerful being could guarantee that the universe would be a certain way, or work a certain way. Similarly, only a being that knew everything could guarantee anything; for only an all-knowing being could know for sure that it was all-powerful and that nothing unexpected would happen. Only an all-knowing being would know which ideas are really true in the first place. Only an infinite being could be everywhere at once and able to control everything. And only an eternal being would have been around since the beginning of the universe (if there was a beginning) and would not disappear from the scene (which would end the guarantee). So, only a being with no lacks or limitations could guarantee that our clear and distinct ideas are true, and make it possible for us to have knowledge, according to Descartes.
Note: all of these ideas about "perfections" were accepted within the Catholic Church in
Descartes' time. By using these ideas, he is showing that his reasoning is in keeping with Church
But, is there really a being that has all of the "perfections" that Descartes describes? Descartes says that there must be: He realizes that he has lacks and limitations, and that he therefore lacks "perfections": he is imperfect (pages 19-20). He believes that the only way an imperfect being such as himself could realize his imperfections and get an idea of perfection would be to get the idea of perfection from a perfect being. Therefore, Descartes says, a perfect being must exist.
Note: This conclusion seems to rest on the assumption that Descartes, or any human, has a perfect idea of perfection, or at least has an accurate idea of what perfection is like. But if we are imperfect beings, do we really have accurate or perfect ideas of perfection? Could we not be mistaken in our conceptions of perfection? Or, if we did get an idea of perfection, could we tell whether it was accurate?
To realize one's lacks is one thing; to be able to grasp what it would be like not to have those lacks is another. For example, I realize that I lack the ability to fly (without an airplane, helicopter, etc.). But this does not mean that I know what it would be like to have the ability to fly. I could imagine flying, but I might imagine it inaccurately.
Now, a person might have some sort of religious revelation of the perfection of the divine. But Descartes says he is writing something that all people can follow and see to be true - not something that only religious revelation will support (page 21, pages 47-50). Therefore questions might be raised as to whether Descartes has proven that a perfect divine being exists. In fact, both religious people (of many faiths) and non-religious people have raised such questions.
However, by using ideas and principles that were accepted by the Catholic Church, Descartes
was able to satisfy many important Church authorities (though not all). He used the Church's
own principles to show that his own conclusions followed.
3. By using principles and ideas that were taught in schools of Catholic theology, Descartes was able to reach conclusions that supported his project. He used the traditional ideas about the perfections of the divine being to show that his method led one to truth, because it led to discovery of clear and distinct ideas.
This had an added bonus: Descartes has not only shown that he is in agreement with some basic
religious principles, but he has also shown (he says) that use of the method results in conclusions
guaranteed by the divine being, AND (he says) that it was use of the method that produced the
proof of the existence of a perfect divine being. So use of the method could not be immoral or
impious, Descartes thinks. And then use of the method in science and philosophy will not be
immoral or impious either. Why not? - Because the method (used carefully) only leads to
conclusions guaranteed by the divine being.
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1.I use the expression 'Judeo-Christian-Islamic divine being' because the description Descartes gives seems to fit not only the beliefs of his own Christian religion, but also those of Judaism and Islam. For example, Descartes describes a single, all-powerful, all-knowing, benevolent divine being, which is consistent with Jewish, Christian (Catholic, Protestant, and Orthodox), and Muslim beliefs. He does not mention the doctrine of the trinity (that the divine being is simultaneously three things and yet one thing) which is specifically Christian and not Jewish of Muslim. Although Descartes wanted to make sure that the Catholic Church authorities would not condemn his work as they did Galileo's, he did not want to limit philosophical and scientific investigation to members of only one faith.