In the First Meditation Descartes (along with the reader) begins his attempt to discover a firm basis for knowledge by introducing his method of doubt and several arguments showing why he feels we should doubt the things we think we know. Descartes’ Method of Doubt sees him rejecting all his previous beliefs before resolving himself to believe only things which can be shown to be certain. Any belief which raises the slightest doubt or uncertainty, regardless of whether it may be shown to be true on closer inspection, should be abandoned as an unsuitable foundation on which to base ones knowledge.
Descartes first applies his Method of Doubt to the information gained from his senses. Descartes points out that it would be unwise to accept as true the information received from his senses, as in the past these have sometimes deceived him regarding “things which are barely perceptible or at a great distance.” One could say the same for hallucinations and optical illusions, which show sense to be uncertain.
However, Descartes then questions whether he can simply disregard everything he knows from his senses, and asks whether it would be reasonable to doubt any empirical propositions, such as that one is sat by the fire. It is at this point that Descartes introduces his dreaming argument to show that something as seemingly certain as this can in fact be open to doubt. Descartes notes that he has previously dreamt of being sat by the fire, and consequently he cannot be certain that he is not again dreaming.
However, dreams, much like paintings, are often made up of representations or likenesses of things which must actually exist in some sense. Although one can dream that one is sat by the fire, one at least knows that the fire exists, or one would not be unable to dream about it.
Furthermore, Descartes explains that we can gain knowledge which is deemed to be more certain when it comes to things such as mathematics. For example, one can know that 2+2=4 and that a triangle has three sides. This is because this is the case whether one is asleep or awake. The argument from dreaming cannot therefore discredit this knowledge.
However, it is possible that one could have a dream in which one thought that a triangle had four sides, for example, so even logical truths come under question with regard to the argument from dreaming.
Descartes then presents a further thought experiment, sometimes known as the evil demon argument, which brings even knowledge of mathematics and geometry in to question. Descartes starts by asking whether it is possible that he is being deceived by God even about things which seem universally true, such as the belief that 2 + 3 = 5.
Furthermore, it seems there is no way of knowing whether his body or indeed the universe exists as God is powerful enough to create the perceptions he has. However such deception would be contrary to the attributes of God who is “said to be supremely good.” After this Descartes supposes that rather than an all good God deceiving him there is instead “some evil demon… who has used all his artifice to deceive me.
It is this last form of doubt which leaves Descartes with no certain knowledge to use as a foundation for his knowledge. From this point of absolute uncertainty where everything is open to doubt, Descartes is then able to pursue his goal of finding certain knowledge from which he can be confident in basing his beliefs on. In ‘Pensées,’ Blaise Pascale attacked his method for resulting in an inability to know anything.
D.J. O’Connor notes that the Method of Doubt could deny that one can understand the process of arguing from premises to conclusion. It follows from this that one cannot attain any knowledge as the assumption that one can deduce a conclusion from premises is uncertain. In trying to reconstruct knowledge in later Mediations, Descartes believes his conclusions are certain as the premises from which they are deduced are certain. What he fails to explain is how the process of deduction itself is certain.