As a simple example, most people assume that their leg is part of them, and the person next to them is not a part of them. But in a truly physical sense, is this even true? One can argue that we can directly control the muscles in our leg, to walk if we want. So given a desire, or a set of neuronal inputs, we can make the nerves that go to our legs fire, and make our legs walk. However, there are 2 main problems with this.
- Some people are paralyzed, and cannot control their legs. So for these people, is their leg not a part of them? This thought experiment refutes the idea that things that we directly control constitute parts of us.
- You can, if you desire, force the nerves to your hands to give the person next to you the middle finger, which will most likely make them walk (away), or perform some other action (kicking you comes to mind haha). This simple example just means to demonstrate that there are other things we can control through our nerves, which we normally don't say are a part of who we are.
So we can control our legs (but not always) through the nerves stemming from our brain, and we can control the legs of someone else (but not always, or not predictably) via the nerves stemming from our brain. So how can we honestly say that our leg is more a part of us than the people around us, such as our friends, enemies, etc.? Even if we say that the densities of atoms between our legs and our brains is much higher than the density of atoms between someone else's legs and our brains, this is does not mean anything. Slight pulls of EM force and gravity from our brain directly affect someone else leg's. Furthermore, any physicist can tell you that what we think of as "attached objects" are not entirely as they seem, as all objects are physically affecting all other objects, but to different degrees and in different ways. So while we can say that our legs are "more" a part of who we are than someone else's legs, there is no definite segmentation.
The next logical step Buddhists take is to say that since everything is interconnected, being compassionate, nice, giving good karma, etc. to others actually helps ourselves. There are many other factors to consider here before this is taken at face value, but it just goes to show things are rarely as they seem. Some Buddhist also claim that all negative emotions (jealousy, lust, anxiety, depression, etc.) stem from not understanding the true nature of reality, but again, this requires further thinking.
It's truly fascinating to understand how other cultures, religions, and people think, and I suppose the most obvious conclusion from all this is to really think critically about our actions and assumptions.