PACKER ON GOD'S IMPASSIVITY

J.I. Packer does an excellent job defining the doctrine of God's impassivity:

"This means, not that God is...unfeeling..., but that no created beings can inflict pain, suffering and distress on him at their own will. In so far as God enters into suffering and grief..., it is by his own deliberate decision; he is never his creatures' hapless victim. The Christian mainstream has construed impassibility as meaning not that God is a stranger to joy and delight, but rather that his joy is permanent, clouded by no involuntary pain.".

[J. I. Packer, "God," in Sinclair Ferguson and David Wright, eds., New Dictionary of Theology (Downers Grove: InterVarsity, 1998), 277] cited from http://www.spurgeon.org/~phil/articles/impassib.htm