A little hand-wavy but I will try to give some sort of definition. If we assume that wealth can be measured with a non-negative real number, then we will set t

A little hand-wavy but I will try to give some sort of definition. If we assume that wealth can be measured with a non-negative real number, then we will set the zero point of the scale at total human extinction. If we set such a zero point, then we will assume that moving away from zero, that is moving away from extinction, is moving in a positive direction. Therefore, anything which contributes to the robustness and stability of the system could be argued as "wealth" or "valuable".