Full Idea
It is naïve to be astonished when we do not stick to firm resolutions. Something stimulated the resolution, but that something was not powerful enough to bring us to the point of carrying it out. Making the resolution may even have exhausted the stimulus.
Gist of Idea
Weakness of will is the inadequacy of the original impetus to carry through the action
Source
Simone Weil (Is There a Marxist Doctrine? [1943], p.169)
Book Reference
Weil,Simone: 'Oppression and Liberty' [Routledge 1955], p.169
A Reaction
Socrates says it is a change of belief. Aristotle says it is a desire overcoming a belief. Weil gives a third way: that it is a fading in the strength of the original belief/desire impetus.