 If we are to avoid mixing homology and analogy, then the appropriate notion of function cannot be based on selection history, which is allied with the concept of analogy and concerns a particular variety of function. Similarly, viability interpretations concentrate on features where the variety of function is critical because of conferred survival advantages. Any interpretation of function that relies on a particular variety of function (because it was selected or because it confers viability) clashes with the demand that homology concern something “under every variety of form and function.” A causal role interpretation emphasizes a systemic capacity to which a function makes a contribution. It too focuses on a particular variety of function, though in a way different from either selected effect or viability interpretations. Only an activity interpretation (‘what something does’) accents the function itself, apart from its specific contribution to a systemic capacity and position in a larger context. Therefore, the most appropriate meaning to incorporate into homology of function is “activity-function” because it is at least possible for activity-functions to remain constant under every variety. An evaluation of sameness due to common ancestry is made separately from the role the function plays (or its use), whether understood in terms of a causal role, a fitness advantage, or a history of  selection.[19] Activity-functions can be put to different uses while being shared via common descent (i.e., homologous). More precisely, homology of function can be defined as the same activity-function in different animals under every variety of form and use-function (Love 2007). This unambiguously removes the tension that plagued functional homology.