Stoic indifferents
One of the most contentious Stoic doctrines is the claim that virtue is the only good. Health, wealth, beauty, pleasure, reputation, noble birth — and their opposites — are neither good nor bad, and thus "indifferent" (adiaphora). Just as what is hot necessarily provides heat, what is good necessarily provides benefit. Wealth and health, however, "neither do benefit nor harm": in no case is a human life made better or worse by possessing or lacking them. Only virtue benefits a human being and makes life go well.
The argument structure
The Stoics do not rest this conclusion on the observation that wealth sometimes benefits and sometimes harms. Their position is categorical: wealth never, in any case, constitutes a genuine benefit. Chrysippus argued this from an account of human nature — our nature is such that only the perfection of reason is genuinely beneficial to us. Other arguments can be found in Cicero (De Finibus) and Seneca's Moral Letters (82, 85, 87).
Preferred and dispreferred indifferents
The mainstream Stoic position — explicitly associated with Chrysippus, against marginal figures who endorsed absolute equality of all indifferents — holds that some indifferents are of a promoted or preferred type (proêgmena / praeposita): they have value (axia), are "according to nature" (kata phusin), and it is appropriate — one's proper function (kathêkon) — to select them over their opposites in normal circumstances. Health, wealth, and reputation are paradigm preferred indifferents; disease, poverty, and disrepute are dispreferred. The virtuous agent is therefore not indifferent in attitude, but actively and conscientiously pursues preferred indifferents — while knowing that acquiring them makes no difference to her happiness.
The ancient dilemma
Ancient critics (Plutarch, Cicero) pressed a dilemma. There are only two ways to explain why the agent should pursue health: either (i) health really is good and contributes to happiness — contradicting the Stoic claim that only virtue is good — or (ii) happiness is not the ultimate goal — contradicting the bedrock eudaimonist thesis. Option (i) means preferred indifferents are just "external goods" by another name; option (ii) abandons the framework altogether. This objection remains the sharpest challenge to the Stoic account.
The epistemic interpretation
The most promising modern response — sketched by Cooper (1999a) and Brennan (2005), and authoritatively presented by Klein (2015) — is that the agent is justified in selecting health over illness not because health is good, but because, for finite agents who lack knowledge of future events, pursuing preferred indifferents is generally the most epistemically responsible way of following the will of Zeus. The telos involves "living in accordance with experience of what happens by nature," and correct selection — the quality of reasoning — contributes positively to happiness even though the acquisition of the indifferent does not. Virtuous selection, like excellent dancing, succeeds even if Zeus does not produce the object pursued. The Stoics also introduced the concept of "reservations" (Stobaeus 65Q), whereby the Sage's selections include a mental clause acknowledging that external outcomes remain outside her control. This connects indifferents directly to the Stoic core practices of focusing on what is up to us and releasing what is not.