Continued

Descartes is probably more influential than most people realize both in terms of defining scientific methodology and in restricting science to be highly deterministic.

Much of his operational advice is to continually define knowledge in terms of doubt:

I distinguish the two as follows: there is conviction when there remains some reason which might lead us to doubt, but knowledge is conviction based on a reason so strong that it can never be shaken by any stronger reason.

First of all, as soon as we think that we correctly perceive something, we are spontaneously convinced that it is true. Now if this conviction is so firm that it is impossible for us ever to have any reason for doubting what we are convinced of, then there are no further questions for us to ask: we have everything that we could reasonably want, For the supposition which we are making here is of a conviction so firm that it is quite incapable of being destroyed; and such a conviction is clearly the same as the most perfect certainty

These passages (and others) clarify that Descartes understands doubt as the contrast of certainty. The requirement that knowledge is to be based in complete, or perfect certainty, amounts to requiring a complete absence of doubt- an inability to undermine one's own conviction. Descartes' methodic emphasis on doubt, rather than on certainty, marks an epistemological innovation by introducing the concept of "method of doubt".

While Descartes is on the right track here, as we would call this "method of doubt" as the scientific process of empirical falsification, he remains steadfastly committed to the notion of the certain truth (because that is what God has created). Moreover, attainment of this certain truth requires a process of "rational insight" (as opposed to true science which is mostly guessing ...). During moments of certainty, it is as if my perception is guided by "a great light in the intellect" (Med. 4, AT 7:59). This rational illumination empowers me to "see utterly clearly with my mind's eye"; my feelings of certainty are grounded, "I see a manifest contradiction" in denying the proposition of which I'm convinced. (Med. 3, AT 7:36)

Implicitly, (or perhaps expliclty), Descartes is not necessarily declaring and absolute Truth here, but rather he is declaring that absolute certainly can be obtained (and there may, in fact, be no difference between these two concepts - this ain't a philosophy class after all).

Of his own methodology, Descartes writes:

Throughout my writings I have made it clear that my method imitates that of the architect. When an architect wants to build a house which is stable on ground where there is a sandy topsoil over underlying rock, or clay, or some other firm base, he begins by digging out a set of trenches from which he removes the sand, and anything resting on or mixed in with the sand, so that he can lay his foundations on firm soil. In the same way, I began by taking everything that was doubtful and throwing it out, like sand … (Replies 7, AT 7:537)

The theory whereby justified beliefs are best structured on an analogy to architecture traces back to ancient Greek thought—to Aristotle, and to work in geometry. That Descartes' method effectively pays homage to Aristotle is, of course, welcome by his Aristotelian audience. However, he views Aristotle's foundationalist principles as incomplete, at least when applied to metaphysical inquiry. I suggest that his method of doubt is intended to complement foundationalism. The two methods are supposed to work in cooperation, as conveyed in the above quotation. Let's consider each method.

Foundationalism

The central insight of foundationalism is to organize one's beliefs in the manner of a well structured, architectural edifice. Such an edifice owes its structural integrity to two kinds of features: a firm foundation and a superstructure of support beams firmly anchored to the foundation. A system of justified beliefs might be organized by two analogous features: a foundation of unshakable first principles, and a superstructure of further propositions anchored to the foundation via unshakable inference.

Exemplary of a foundationalist system is Euclid's geometry. Euclid begins with a foundation of first principles—definitions, postulates, and axioms or common notions—on which he then bases a superstructure of further propositions. Descartes' own designs for metaphysical Knowledge are inspired by Euclid's system:

Those long chains composed of very simple and easy reasoning, which geometers customarily use to arrive at their most difficult demonstrations, had given me occasion to suppose that all the things which can fall under human knowledge are interconnected in the same way. (Discourse 2, AT 6:19).

It would be misleading to characterize the arguments of the Meditations as unfolding straightforwardly according to geometric method. But Descartes maintains that they can be reconstructed as such, and he expressly does so at the end of the Second Replies—providing a “geometrical” exposition of his central constructive steps, under the following headings: definitions, postulates, axioms or common notions, and propositions (AT 7:160ff).

As alluded to above, the Meditations contains a destructive component that Descartes likens to the architect's preparations for laying a foundation. Though the component finds no analogue in the method of the geometers, Descartes appears to hold that this component is needed in metaphysical inquiry. The discovery of Euclid's first principles (some of them, at any rate) is comparatively unproblematic: such principles as that things which are equal to the same thing are also equal to one another (one of Euclid's axioms) accord not only with reason, but with the senses. In contrast, metaphysical inquiry might have first principles that conflict with the senses:

The difference is that the primary notions which are presupposed for the demonstration of geometrical truths are readily accepted by anyone, since they accord with the use of our senses. Hence there is no difficulty there, except in the proper deduction of the consequences, which can be done even by the less attentive, provided they remember what has gone before. … In metaphysics by contrast there is nothing which causes so much effort as making our perception of the primary notions clear and distinct. Admittedly, they are by their nature as evident as, or even more evident than, the primary notions which the geometers study; but they conflict with many preconceived opinions derived from the senses which we have got into the habit of holding from our earliest years, and so only those who really concentrate and meditate and withdraw their minds from corporeal things, so far as possible, will achieve perfect knowledge of them. (Replies 2, AT 7:156-57)

Among Descartes' persistent themes is that such preconceived opinions can have the effect of obscuring our mental vision of innate principles; THIS IS OBSERVER BIAS that where there are disputes about first principles, it is not “because one man's faculty of knowledge extends more widely than another's, but because the common notions are in conflict with the preconceived opinions of some people who, as a result, cannot easily grasp them”; whereas, “we cannot fail to know them [innate common notions] when the occasion for thinking about them arises, provided that we are not blinded by preconceived opinions” (Prin. 1:49-50, AT 8a:24). These “preconceived opinions” must be “set aside,” says Descartes, “in order to lay the first foundations of philosophy” (1643 letter, AT 8b:37). Unless they are set aside, we're apt to regard, as first principles, the mistaken (though prima facie obvious) sensory claims that particularists find attractive. And mistakes in the laying of foundations weaken the entire edifice. Descartes adds:

All the mistakes made in the sciences happen, in my view, simply because at the beginning we make judgements too hastily, and accept as our first principles matters which are obscure and of which we do not have a clear and distinct notion. (Search, AT 10:526)

Though foundationalism brilliantly allows for the expansion of knowledge from first principles, Descartes thinks that a complementary method is needed to help us discover genuine first principles. He devises the method of doubt for this purpose—a method to help “set aside” preconceived opinions.

2.2 Method of Doubt

Descartes opens the First Meditation asserting the need “to demolish everything completely and start again right from the foundations” (AT 7:17). In the architectural analogy, we can think of bulldozers as the ground clearing tools of demolition. For Knowledge building, Descartes construes sceptical doubts as the ground clearing tools of epistemic demolition. Bulldozers undermine literal ground; doubt undermines epistemic grounds.

Descartes' ultimate aims, however, are constructive. Unlike “the sceptics, who doubt only for the sake of doubting,” Descartes aims “to reach certainty—to cast aside the loose earth and sand so as to come upon rock or clay” (Discourse 3, AT 6:28-29). Bulldozers are typically used for destructive ends, as are sceptical doubts. Descartes' methodical innovation is to employ demolition for constructive ends. Where a bulldozer's force overpowers the ground, its effects are destructive. Where the ground's firmness resists the bulldozer's force, the bulldozer might be used constructively—using it to reveal the ground as firm. Descartes' innovation is to use epistemic bulldozers in this way, using sceptical doubts to test the firmness of beliefs put forward as candidates for the foundations of Knowledge—testing their epistemic shakability.

Consider first the universal character of methodic doubt. In urging a universal doubt, Descartes does not mean simply that we're to apply doubt to all candidates for Knowledge. He is urging something much stronger. He means that in the initial demolition phase of the project we're to apply doubt collectively, undermining the candidates for the foundations of Knowledge all in one go: it is necessary “to demolish everything completely and start again right from the foundations” (Med. 1, AT 7:17). Why must doubt be universal to this extent? Descartes offers the following analogy:

Suppose [a person] had a basket full of apples and, being worried that some of the apples were rotten, wanted to take out the rotten ones to prevent the rot spreading. How would he proceed? Would he not begin by tipping the whole lot out of the basket? And would not the next step be to cast his eye over each apple in turn, and pick up and put back in the basket only those he saw to be sound, leaving the others? In just the same way, those who have never philosophized correctly have various opinions in their minds which they have begun to store up since childhood, and which they therefore have reason to believe may in many cases be false. They then attempt to separate the false beliefs from the others, so as to prevent their contaminating the rest and making the whole lot uncertain. Now the best way they can accomplish this is to reject all their beliefs together in one go, as if they were all uncertain and false. They can then go over each belief in turn and re-adopt only those which they recognize to be true and indubitable. (Replies 7, AT 7:481)

Because one bad apple can spoil the whole bunch, the only sure means to a rot-free basket is to discard the whole lot. What Descartes notices is that even one falsehood that is mistakenly regarded as a genuine first principle—say, the belief that the senses are reliable, or that ancient authorities should be trusted—threatens to spread falsehood to other beliefs in the system. A collective doubt helps avoid such mistakes. It ensures that the method only approves candidate first principles that are unshakable in their own right: it ensures that the appearance of unshakability in a candidate is not owed to its logical relations to other principles, themselves not subjected to collective doubt. Yet in acknowleding the correct role or "collective doubt" in science, Descartes still makes no allowance, like Cusa has made earlier, than there is not such thing as absolute certainty or aboslute truth

A potential problem remains. Does not the problem of the “light-duty bulldozer” repeat itself? No matter how firm one's ground, it would be dislodged in the face of a yet bigger bulldozer. This raises the worry that there might not be unshakable ground, but only yet unshaken ground. Descartes' goal of utterly indubitable epistemic ground may simply be elusive.

This elusiveness would be a scientific way of looking at the problem but Descartes argues his way out of this by appealing to God as the creator of essentially physics.

I see that the certainty of all other things depends on this [knowledge of God], so that without it nothing can ever be perfectly known [perfecte sciri]. (Med. 5, AT 7:69)

[I]f I did not possess knowledge of God … I should thus never have true and certain knowledge [scientiam] about anything, but only shifting and changeable opinions. (Med. 5, AT 7:69)

And upon claiming finally to have achieved indefeasible Knowledge:

Thus I see plainly that the certainty and truth of all knowledge [scientiae] depends uniquely on my awareness of the true God, to such an extent that I was incapable of perfect knowledge [perfecte scire] about anything else until I became aware of him. (Med. 5, AT 7:71)

These texts make a powerful case that nothing can be indefeasibly Known prior to establishing that we're creatures of an all-perfect God.

In the end, Descartes sets up the possibility that uncertainty could dominate (first passage) but then quickly resolves this dilemma by declaring that God necessarily exists (second passage)

And since I have no cause to think that there is a deceiving God, and I do not yet even know for sure whether there is a God at all, any reason for doubt which depends simply on this supposition is a very slight and, so to speak, metaphysical one. But in order to remove even this slight reason for doubt, as soon as the opportunity arises I must examine whether there is a God, and, if there is, whether he can be a deceiver. For if I do not know this, it seems that I can never be quite certain about anything else. (Med. 3, AT 7:36)

I have perceived that God exists, and at the same time I have understood that everything else depends on him, and that he is no deceiver; and I have drawn the conclusion that everything which I clearly and distinctly perceive is of necessity true. … what objections can now be raised? That the way I am made makes me prone to frequent error? But I now know that I am incapable of error in those cases where my understanding is transparently clear. … And now it is possible for me to achieve full and certain knowledge of countless matters, both concerning God himself and other things whose nature is intellectual, and also concerning the whole of that corporeal nature which is the subject-matter of pure mathematics. (Med. 5, AT 7:70-71)