Full Idea
Augustine insisted that 'ought' does not, in any straightforward way, imply 'can' - which distinguishes him from most modern ethicists.
Gist of Idea
Augustine said (unusually) that 'ought' does not imply 'can'
Source
report of Augustine (works [c.415]) by Gareth B. Matthews - Augustine p.74
Book Reference
'Shorter Routledge Encyclopaedia of Philosophy', ed/tr. Craig,Edward [Routledge 2005], p.74
A Reaction
Not unreasonable. I ought to help my ailing friend who lives abroad, but I haven't the time or money to do it. We can experience impossibilities as duties. Impossibilities are just excuses. Augustine is opposing the Pelagian heresy.