Aristotle's Ergon Argument

Rational activity is the human function.

Therefore,

Rational activity is the activity peculiar to humanity.

$P$ can be a good human if and only if $P$ can produce virtuous rational activity.

The function of $x$ is the activity or activitiesies peculiar to $x.$

Therefore,

In general, if $x$ has a function $E,$ then something is a good $x$ if and only if it produces virtuous $E.$