 Malebranche is a philosopher whose thought developed markedly over time. In part this was due to his lively engagement with critics and his attempts to respond to their objections, and also to his recognition that his doctrines required further articulation than he had provided in earlier writings. One of the most significant developments to the theory of Vision in God was the view that ideas are causally efficacious. Malebranche had already argued that ideas have properties – necessity, eternality, immutability, universality, etc. – but in response to an objection by his Cartesian critic Pierre-Sylvain Régis, he came to ascribe causal powers to them as well. This new theory is introduced in the context of the claim that the mind is united to God, even in this life. It is because of this union, Malebranche maintains, that Vision in God is possible. We are always in immediate contact with being in general and so can focus our attention on the idea of intelligible extension at any time or immediately perceive it in sense perception.