"I think therefore I am" epitomizes what is wrong with humanity as a whole: we are constantly under the impression that we physically exist. Thinking is all relative to the mind and not the body itself - our visual perception is riddled with nothing but lies. How can I assure myself that if I look at a pillow that someone else sees the same exact pillow in regard to shape, color, size, and other physical attributes? 

We can't - our own mere perceptions are limited to our own minds. The truth of it all is that we're all the figments of someone's imagination ... and that particular someone is the figment of another man's imagination and so on and so forth. We are entrapped in an endless loop of despair that we shall never escape from and that's the cold hard truth. The people we conjure in our minds, the people and places we PHYSICALLY see with our eyes, the entire world that stands in front of us are all deceiving us from the truth of the real world. 

Descartes alludes to substance dualism, which emphasizes the separation of both physicality and mentality. Our mentality cannot be without our physicality and our physicality cannot think without our mentality. Our senses (touch, taste, hearing, sight, and smell) all fall under the category of physicality - we PHYSICALLY see something (like seeing your wife in lingerie) as opposed to MENTALLY seeing something (imagining your wife in lingerie). Due to this separation, we cannot assume that our brains are responsible for both the mental and the physical aspects - we can in fact all be brains or unknown entities floating in glass jars in some greater power's laboratory. Our brains could simply be fillers or chains to what actually is controlling us - a brain that is stimulated whether with a probe or damage from an outside source could instantaneously affect what actually is controlling us in the nonphysical world. 

Everything we see, hear, taste, touch, and smell are mirages and thus alludes to the fact that we are in fact mirages as well. God could very well exist - perhaps the higher power above us initiated the Big Bang years and years ago to thus spur our very so-called "existence." Cancer, AIDS, and other ailments could have been inflicted upon us by forced injection from the greater power to allow the greater power to experiment with us. We are all lab rats and those who worship God and praise the existence of God are just as prone to disease and other catastrophic things as much as nonbelievers. Love is forced - the definition of a "soulmate" and "true love" both may very well exist (divorces and the acts of cheating on a spouse are most likely experimental groups in order to fully express the legitimacy of true love). Emotions are also forced - people cry during funerals and people may laugh during funerals. People get angry at their slumping sports team while others cherish their prosperous sports team. 

... and who knows, our greater power could be under the control of an even GREATER power. We could be on the very bottom of this chain - we are the untouchables, the inferiors, the entities who witness both birth and death. We are imaginary scum, being constantly toyed by our greater power.