Definition
In Stoic ethics, indifferents are everything other than virtue and vice. Health, wealth, reputation, comfort, and their opposites all fall into this class. They are called indifferent because, on their own, they neither make a life good nor make it bad — only the quality of one's character does that.
Indifferent does not mean unimportant or that one should ignore them. It means they are not the substance of flourishing.
Why it matters
How it works
The Stoics refined the category by distinguishing preferred and dispreferred indifferents. Health, wealth, and good repute are naturally preferred — reasonable to choose when nothing more important is at stake. Illness, poverty, and disrepute are dispreferred. Yet preference is not need: a wise person selects the preferred indifferent but remains undisturbed if it cannot be had.
This lets Stoicism be practical without being materialist. One acts in the world, makes sensible choices, and yet keeps flourishing anchored where it cannot be lost.