The argument from function. A short syllogism about toil (πόνος — "hardship, effort, pain"). Premise: for an organ, toil is not against nature so long as the organ does its own work — the foot walks, the hand grasps. Effort is the very carrying-out of the function, not a violence done to it. The governing discipline is desire (how to regard hardships); the sustaining one is action (to do "what belongs to a human").
Transfer to the human being. By analogy: for a human as human, toil is not against TERMnature so long as he does τὰ τοῦ ἀνθρώπου — "the things of a human" (the rational, social, just). Labour rooted in one's own function is not a distortion of nature but its working.
Conclusion: not an evil. The final step is ethical: "if not against nature, then not an TERMevil" (κακόν). Here the Stoic axiom operates: evil is only what is contrary to the nature of a rational being — that is, vice; whereas hardship, being κατὰ φύσιν, falls into the class of the indifferent — hard, but not bad. Thus the complaint about the "weight" of duty is dissolved: the fatigue of right action does not belong to evil.
Parallels. DOGMALiving according to nature as the criterion; Med. 8.46 (what is according to nature is bearable); Med. 10.33 (to do what is "according to the constitution of man"); 06-02 (cold, heat — no obstacle to "doing what is fitting"). Cf. the image of the organ doing its own work in 02-01 (but there — of social cooperation; here — of function and toil).