The matter that is consumed during a lifespan is what determines individual existance. It might be hard for some people to understand this, but most people can agree with this statement if they are given a good explaination. An easy, and well known, summary of this is "you are what you eat". As most people know, everything that exists in the universe is made up of matter. An object must have mass to exist, and everything that has mass is matter, by definition. That means that everything around people is matter, including the people themselves. The way people, and other creatures, gain and replace matter is through consumption. The thing that seperates one individual from another is the fact that they are made up of a seperate collection of matter. Even though twins or a clone are organically identicle, they are still seperate beings that inhabit their own space. (Here comes the hard part) Now, we all start life as a one-celled organism. After fertilization we connect to the uterus, and start our life-long process of consumption. The cells start to divide, and a complex organism is formed from the schematics of our DNA. Well, those first few cells do not determine existance. In fact, those same cells are not part of our bodies right now, because the matter has been discarded as waste a long time ago. The body that exists as a person right now is just a built clone of that original DNA schematic in that cell. The thing that determines existance is what a person is made up of at any one time. A person is just the things that he or she has consumed up to any point in his or her life. Let's say that over a person's life he or she consumed a whole seperate set of matter other than what he or she has actually consumed. Would the same person's consiousness still exist? The answer is, no. If the person is made up of a different set of matter than it really is, then it would be along the lines of a twin of that person. A duplicate person would still exist in the world in the place of the original, and even have the same thoughts, beliefs, and reactions. Although, there would basically be somebody else who recieves the stimulus, and controls the body. Another way to show this effect is to clone a person's brain with all memories and thoughts intact, and then replace the original brain with the cloned one. If the person could possibly recover, he or she would wake up, and seem like they were same person as before. It still remains that the original person is not really there, and only a copy exists in their place. This would be the same as if you consumed different matter during your life. If you brain was made up of a seperate set of matter then you wouldn't exist. A person that is exactly like you would exist in your place. On a lesser note. There is also something that goes with this. Scientists found that your brain cells actually die and are replaced by new ones over the coarse of your life. Eventually, by the time you are 60 or so, every cell in your brain has died and been replaced. Would this mean that when you reach an older age the person that you were at age 14 is actually dead, and only a copy of that person exists today? So basically you die, and are replaced slowly over time. Well, that is all I have been thinking about lately. Some people might not agree with this, but I ask that you keep an open mind about it. If you would like me to clearify any of this just say so. I have found it hard to explain this to some people. A couple of people just have a hard time understanding this stuff.