Semipelagianism (hence "SP")--developed by John Cassian in response to Augustine's polemic against Pelagius--implies that one makes a free first step toward salvation, a first step that is in the power of the individual apart from grace. That first step in itself is incomplete, and can be completed only with God's assistance by means of grace.
It seems that SP implies
(A) there can be human actions apart from God's grace,
and that is a proposition I wish to deny. No aspect of human action is possible apart from grace. Insofar as there is an aspect of human action--moral or otherwise--it owes its reality to God's act of creation. But God's act of creation is one of grace--it is a sheer gift. However, since SP implies (A), and (A) is--so far as I can tell--false, it follows SP is false.