"An evil genius, as clever and deceitful as he is powerful,

who has directed his entire effort to misleading me."

How can we know if we are brains in vats?

Can we be sure that we are not deceived by an evil genius?

 In Descartes' First Meditation, Descartes writes that he has

come to the conclusion that many of the opinions he held in his

youth are doubtful, and consequently all ideas built upon those

opinions are also doubtful.  He deduces that he will have to

disprove his current opinions and then construct a new foundation

of knowledge if he wants to establish anything firm and lasting in

the sciences that is absolutely true.  But 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.  He does believe,

however, that whatever can not 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 (eg.

a square tower may look round from far away).  Also, because he

realizes that there are no definitive signs for him to distinguish

being awake from being asleep, he concludes that he can not trust

his judgement to tell him whether he is awake or asleep.  But

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, all-powerful

and all-clever, who has directed his entire effort at deceiving

Descartes by putting ideas into Descartes' head.

Descartes imagines that God is not the one who is deceiving him,

but none other than a malevolent demon, who with deceitful power,

implants false beliefs, " I will suppose therefore that not God,

who is supremely good and the source of truth, but rather some malicious demon

of the utmost power and cunning has employed all his energies in order to deceive me"

When determining what is open to doubt, Descartes' evil demon hypothesis conveniently

creates a being who is omnipotent and who uses the power solely to deceive.

What Descartes achieves is making problematic a host of ideas he entertains as products of reason ,

opposed to products of the senses.

"Could not an all-powerful demon make me believe those propositions are true when,

as a matter of fact, they are not?" The deceiver God does not succeed if the person

accepts that the reality he lives in is true. However, with the rise of skepticism and questioning

the veracity of whether the world we live in is accurate or not, perhaps the demon has won after all.

Descartes then leaves the first meditation in a state of confusion. He knows at least how things seem

to appear to him, even if he has no idea how they really are

"I am like a prisoner who is enjoying an imaginary freedom while asleep,

he dreads being woken up, and goes along with the pleasant as long as he can".

Descartes clearly refocused metaphysical thinking into the physical world,

by turning it toward the natural world. His basic structure has four uses of doubt,

firstly to free us from preconceived opinions or prejudice,

the second is to lead the mind away from the senses,

the Cartesian Doubt third use of doubt makes it impossible to have any further doubts

about those things which alter such an "extensive doubt" and are discovered to be true,

while the fourth is to provide us with an understanding of what certainty is.

Descartes methodological doubt can be defined as foundationalism,

which is the belief that knowledge is formed on different levels, much like an inverted pyramid.

Such that, complex beliefs come first, then beneath that are simpler beliefs

and beneath them are the simplest beliefs. Foundationalism requires not only this hierarchy effect,

but also that nothing is accepted as knowledge until we know upon what it is based.

With these list of doubts, each progressively more broad,

Descartes finally is satisfied that he has sufficiently disproved

his previous opinions.  He now is ready to build a new foundation

of knowledge of a physical world (the real world) based on what

must absolutely be true.

A modern version of Descartes' skeptical view of universe is imagined in "brains in vats" model. 

What if an evil genius, a scientist with the intention of deceiving you,

without your knowing, removes your brain and puts it in a vat with the necessary chemicals to keep your

 brain cells alive. Then this scientist connects your nerve endings to a supercomputer in such a way

that your brain has the "experience" that it is living an ordinary life with its smells, tastes, visions, etc.   

Let's say that your illusion felt just as real. How could you know that you're a brain in a vat? 

How can you distinguish the illusion from what is real?