In Descartes’ First Meditation, he writes that he has come to the conclusion that many of the opinions he held in his youth are doubtful. Consequently, all ideas built upon those opinions are also doubtful. He deduces that he will have to disprove his current opinions and construct a new foundation of knowledge if he wants to establish anything firm and lasting in the sciences that is absolutely true. Rather than disprove each of his opinions individually, Descartes attacks the principles that support everything he believes with his Method of Doubt.
The Method of Doubt is Descartes’ method of fundamental questioning in which he doubts everything that there is the slightest reason to doubt. It should be mentioned that Descartes does not necessarily believe that everything he doubts is true. However, he does believe that whatever cannot be doubted for the slightest reason must be true. Descartes spends Meditation One trying to disprove his fundamental beliefs.
First, Descartes doubts that his senses are generally trustworthy because they are occasionally deceitful. For example, a square tower may look round from far away. Additionally, he realizes that there are no definitive signs for him to distinguish being awake from being asleep. Therefore, he concludes that he cannot trust his judgement to tell him whether he is awake or asleep. However, whether he is asleep or awake, arithmetic operations still yield the same answer and the self-preservation instinct still holds. To disprove these, Descartes abandons the idea of a supremely good God, like he has believed in all his life, and supposes an evil genius who is all-powerful and all-clever. This genius has directed his entire effort at deceiving Descartes by putting ideas into his head.
With these three main doubts, each progressively broader, Descartes is finally satisfied that he has sufficiently disproved his previous opinions. He is now ready to build a new foundation of knowledge of the physical world (the real world) based on what must absolutely be true. However, Berkeley would argue that Descartes is wasting his time by trying to discover what must be absolutely true in the real world. In his Dialogue One, Berkeley argues that there is no real world and that all sensible objects (those which can be immediately perceived) exist only in the mind. He starts by proving that secondary (extrinsic) qualities exist only in the mind by using the Relativity of Perception Argument.
As an example, Berkeley writes that if you make one of your hands hot and the other cold, and put them into a vessel of water, the water will seem cold to one hand and warm to the other. Since the water cannot be warm and cold at the same time, it must follow that heat (a secondary quality) must only exist in the mind. Berkeley also uses the qualities of taste, sound, and color as examples to prove that all secondary qualities must reside in the mind. However, Berkeley also says that the same argument can be applied to primary (intrinsic) qualities. He writes that to a mite, his own foot might seem a considerable dimension, but to smaller creatures, that same foot might seem very large.
Since an object cannot be different sizes at the same time, it follows that extension must exist only in the mind. Further, since all other primary characteristics cannot be separated from extension, they too must exist only in the mind. An interesting aspect of Descartes’ Dualistic view and Berkeley’s Idealistic view is the necessity of God. Descartes needs an all-good non-deceiving God to ensure that the ideas of primary qualities of objects he perceives in his mind accurately represent those qualities of objects in the external world. In the Third Meditation, Descartes says that God is infinite and finite is the lack of infinite.
Infinite, he says, is not the lack of finite. Since our concept of the infinite could not have come from the concept of the finite (since infinite is not the lack of finite), the idea of infinite could only have come from God. This proof is shaky at best. Berkeley, on the other hand, needs God to give us the ideas of the objects we see since there is no physical world to draw those ideas from through the senses. But rather than proving God to prove his philosophy, Berkeley uses his philosophy as the proof of God’s existence. In his Second Dialogue, Berkeley says God must exist to put the same real ideas into everybody’s minds because minds cannot interact directly.
However, if it were the case that God did not actually exist, or had used his infinite powers to remove his infinity after he created the universe because he was no longer needed, both Descartes and Berkeley would find their philosophies in trouble.