Don't know why, but I find myself in an academic mood today. Please pardon me for these feeble attempts at explaining things that are too profound for a novice to declare command over. Nevertheless, it can be seen as an interesting challenge to be able to put the terms in a layman's language.
Cogito, ergo sum (I think, therefore I am) .This is a logical statement. It's not your regular wisecrack or an inspirational saying like "I believe, so I will. ", but a conclusion arrived at with much deliberation and analysis. The great philosopher Descartes spent years in seclusion, clearing up his mind of previously acquired wisdom, to come up with his Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences.
The mindless adoption of the statement in puns and quips on TV and in everyday life goes to show how knowledge/things are taken for granted once they are created and ready to use. A classical example is "Zero". It took millenniums and civilizations, before "Zero" was "invented" and now we find it hard to even imagine that "Zero" was something which needed to be "invented”. To us, it now appears to be such a trivial concept that we put it in the syllabus for kindergartens. Please do read this link to appreciate the sweat gone into creating "Zero".
Returning to the topic of the day, why does "I think" conclude that "I am”? Why didn't he make a statement like "I breathe, therefore I am"? To get the answers, one must first know what the need for the statement was.
One of my friends is partially color-blind. The day we discovered his "condition", we were arguing over the color of a flag on a temple about half a km away; he claimed it was green while to me it was orange. Thankfully enough, a couple of other guys joined us and we won 3-1. Later on, that friend had a check-up done and the "condition" was diagnosed.
Still, the whole episode left me wondering, what if someone identifies a color as green, but the shade he perceives is what looks orange to me; and vice versa. The problem with my friend was that he saw both of them as one color which he called green. However, it seemed perfectly plausible that some other person perceives the color-spectrum in a different way like the way we do when we wear colored goggles; what if someone would have put a colored goggle on me right from the time I was born. I wouldn't have been diagnoised with color-blindness since I would have still distinguisehd between all the colors; just that the perception would have been different. Is there one "true" perception?
Descartes noticed many such inconsistencies around him. Then there were questions like what if reality as it appears to us is a dream too. Am I just a part of someone else's dream or imagination? Or things and people around me are nothing but creation of my mind.
So he decided to build a system based on reasoning alone and devised a few rules to help him do that. One of the most important rules is known as "Cartesian Doubt", which, in his own translated words, is to "reject as absolutely false everything concerning which I could imagine the least doubt to exist.” This explains why he cannot start with "I breathe, therefore I am". To make this statement, one must first define what is breathing. That would involve defining nose, air, lungs, life etc. Not an ideal way to begin. He doubted everything he perceived and nose was definitely amongst them.
Once again he beautifully explains: “considering that all the thoughts we have when awake can come to us also when we sleep without any of them being true, I resolved to feign that everything which had ever entered my mind was no more truth than the illusion of my dreams. But I observed that, while I was thus resolved to feign that everything was false, I who thought must of necessity be somewhat; and remarking this truth--I think, therefore I am--was so firm and so assured that all the most extravagant suppositions of the skeptics were unable to shake it, I judged that I could unhesitatingly accept it as the first principle of the philosophy I was seeking."
So there. That's why it is "I think" and not "I breathe" or "I sleep". Thinking is the first and only activity that is self-evident,verifiable and not needing a definition.
One must be careful here; the above reasoning does not allow me to conclude that Priyanka Chopra exists because she thinks. For me, she is an external being, a perception and hence may be part of an illusion. A lot more reasoning and arguments than the one involved in reaching this first principle is required to even start considering Priyanka Chopra's existence. However, when and if she is thinking, then she too can be sure of her existence only. I am afraid that even after establishing further principles, I won't be existing for her.
Another point to be noted here is that the reasoning involved to reach the statement, or the first principle as it is called, just means that "I" that is proven to be existing here is something that is thinking and not necessarily the "Prashant Dhanke" as I know him to be; in fact that existing, thinking thing is not even established to be a human being. Many more principles need to be established and reasoned before such complex statements can be discussed.