Complex sentence: As such, this was also a rejection of the Ontological Proof of God's existence and also Hegel's Absolute Idealism, with its Absolute Spirit, both of which claimed that something exists, or has being, because it is a thought in someone's mind.
Split into simpler sentences: As such, this was also a rejection of the Ontological Proof of God's existence and also its equivalent, Hegel's Absolute Idealism, with its Absolute Spirit. Both claimed that something exists, or has being, because it is a thought in someone's mind.