The only way of defining social mobility is if one can determine social position. The mobility would therefore depict changes in position. However position is relative to the context. Therefore, social mobility can be judged at an individual level, but if one's mobility is ascending, it must be in detriment of another person.
The challenge is to define therefore, the social position without resorting the measuring individual wealth. For example, we can judge access to food, health, education. In this way, everyone in a given society (including the whole world) can have ascending mobility, even though their relative positions do not change.
This opens the second part of my concerns. Once the mobility is ascending, the relative position is meaningless unless it is for prejudice.
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