Extensionality

Extensionality is a concept that says that two things that have the same parts are the same. It is like the saying, "if it looks like a duck, swims like a duck and quacks like a duck, then it must be a duck". Extensionality is saying that if something has all the same parts, then that thing is the same. For example, if you and your friend both have two eyes, two ears, a nose and a mouth, then you and your friend are the same, even though you might look different.