Full Idea
'One ought to be moral' makes no sense at all unless the 'ought' has the moral subscript, giving a tautology, or else relates morality to some other system such as prudence or etiquette.
Clarification
the moral subscript must say 'according to morality'
Gist of Idea
Saying we 'ought to be moral' makes no sense, unless it relates to some other system
Source
Philippa Foot (Morality as system of hypothetical imperatives [1972], p.169 n18)
Book Reference
Foot,Philippa: 'Virtues and Vices' [Blackwell 1981], p.169
A Reaction
This aims to undercut the Kantian view that morality is an absolute call to duty (filling us with wonder, like the starry heavens). Foot aims to root morality in the real world.