Full Idea
Someone has 'internal reasons' to act when the person has some motive which will be served or furthered by the action; if this turns out not to be so, the reason is false. Reasons are 'external' when there is no such condition.
Gist of Idea
Reasons are 'internal' if they give a person a motive to act, but 'external' otherwise
Source
Bernard Williams (Internal and External Reasons [1980], p.101)
Book Reference
Williams,Bernard: 'Moral Luck: Papers 1973-1980' [CUP 1981], p.101
A Reaction
[compressed] An external example given is a family tradition of joining the army, if the person doesn't want to. Williams says (p.111) external reason statements are actually false, and a misapplication of the concept of a 'reason to act'. See Idea 8815.
Related Idea
Idea 8815 Belief externalism is false, because external considerations cannot be internalized for actual use [Pollock]