Virtue

In plain terms

A rule tells you what to do. A virtue is what makes you the kind of person who does it without needing to be told, and who can tell when the rule does not fit.

That second part is why virtue is not a decorative addition to a system of rules. Rules run out. They conflict, they fail to anticipate the case, and applying them requires judging which one applies - which is not itself a rule.

And virtues are acquired the way skills are: by doing the thing, badly at first, under correction, until it becomes what you would naturally do. Nobody becomes honest by learning that honesty is required. Aristotle's word for the process is habituation, and his point is that character is built by practice rather than delivered by instruction.

As a philosopher would put it

Aristotle defines virtue as a stable disposition concerning choice, lying in a mean relative to us, determined by reason as a person of practical wisdom would determine it. It requires acting for the right reason, with the right feeling, in the right circumstances - so someone who does the right thing reluctantly is continent rather than virtuous.

The unity of the virtues is the standing structural claim: for Socrates and the Stoics you cannot have one fully without the others, since each requires the practical wisdom that governs all. Aristotle holds a weaker version through phronēsis.

The modern revival dates from Anscombe's 1958 argument that moral philosophy should be abandoned until an adequate psychology is available, and that obligation-based ethics survives as the residue of a lawgiver no longer believed in. MacIntyre's After Virtue develops this historically. The situationist challenge, drawing on Milgram and the Darley-Batson study, holds that behaviour is driven by circumstance rather than by traits - a challenge partly blunted by evidence on the replication of those studies but not disposed of.

What is actually disputed

Whether character traits exist as the theory needs them to.

The situationist challenge is that behaviour varies with circumstance far more than with any stable disposition - people help when unhurried and not when late, obey when the setting is authoritative, and are poorly predicted by their reputations. If traits are that fragile, an ethics built on them is built on something that may not be there.

Defenders reply that virtue was never claimed to be common, that Aristotle regarded full virtue as rare and hard-won, and that the experiments measure single situations rather than settled character. Some of the classic studies have also fared badly in replication, though the general finding of situational influence has not gone away.

A second objection is that virtue ethics gives no action guidance - telling someone to do what a virtuous person would do is unhelpful to anyone not already virtuous. The standard reply is that no theory escapes this, since applying a rule also requires judgement.