People are different from each other. For some scholars and thinkers, these differences don't have any important meaning. They are just variations on an underlying unity and fundamental similarity.
Sure, everyone has a different hair color; the important thing is that everyone has hair.
Sure, some people are short and some are tall, and a few are over seven feet tall. But what matters is that the average height is five foot nine and most people are a little above or below that.
Sure, everyone has different ideas about personal space. But 12 inches feels right to me, and if people deviate from that, then they are just bad people and don't deserve my attention.
Anthropology is always trying to unsettle these kinds of views. When it makes claims about being a human, it starts from the view that humanity is defined by its diversity, and that there is no “normal” or “correct” way to be human.
Many kinds of differences are continuous variations rather than categorical contrasts. Height for instance is an example of continuous variation.
While there are lots of ways in which individual people are different from each other, many of these differences are continuous variation. This means that people of the same community can be as different from people from different parts of the world. Many of these differences are random and specific to individuals, and even if they are not really random, they are determined by so many different causes that there will always be a fuzziness to them in specific individuals' cases.
Many social sciences are interested in looking at differences among people objectively as something that they can measure and quantify as the value of a variable. How much money someone earns in a year, how many years of education one has, how old one are all sociological variables that people can measure numerically. Gender, race, ethnicity, nationality are all categorical sociological variables, and one can sort people into categories based on a particular definition of each concept. This approach to understanding people's lives looks at people from the outside, and sets aside what they think about who they are.
Anthropology is not opposed to this quantitative and objective approach, but it does look at things differently.
Anthropology is interested in an approach to difference which takes a qualitative and symmetrical view of people's acquired (learned) ways of thinking
First, for anthropology, the most important differences are learned (or acquired) from one's social environment. Natural variation is just random noise at the individual level. Most people in one community acquire the same basic knowledge.
Second, the knowledge one acquires from one's society is mostly a way of thinking and of classifying the world. Each society gives its members a distinct way of telling differences, or sorting things (and people) into categories.
People in Australia learn to sort people into the categories of race, ethnicity, and nationality.
People in Auhelawa learn to sort people into the (totemic) categories of birds, Magisubu (wedge-tailed eagle), Ao'ao (crow), Gegela (parrot), etc.
Third, when people acquire their society's distinct way of seeing the world, that this their culture, it becomes a second nature. They don't have to think consciously about how they classify things, because everyone in their community has the same shared knowledge. Acquired knowledge also sets a standard for what counts as normal for the people who acquire it.
Fourth, and finally, people are different from each other but their acquired differences are always relative. What makes you different from an Auhelawa person also makes an Auhelawa person different from you. Anthropology wants to put people's acquired characteristics in a context in which their ways of seeing and thinking are normal, and the observer's way of seeing and thinking stands out as abnormal.