Many biological theories begin with genes, metabolism, information, reproduction, or evolution.
APS begins with agency because agency identifies the activity through which living systems sustain the conditions of their own continued persistence.
Genes, metabolism, development, adaptation, cognition, and evolution are all important biological phenomena. APS does not deny their significance. Instead, it asks what unites them.
The answer is viability-oriented activity. Living systems continuously regulate, repair, adapt, reorganise, and coordinate themselves in ways that preserve viability across changing conditions.
Agency names this activity.
For APS, agency is therefore not one biological phenomenon among many. It is the defining activity through which organised persistence becomes possible.
APS Principle: Life is best understood not by what living systems are made of, but by what they continually do to remain viable.