 This unified account may seem incompatible with Avicenna’s use of a distinction between “true” and “auxiliary” efficient causes. On his account of this distinction, however, the term “auxiliary” merely describes a true efficient cause insofar as its action aids that of another true efficient cause (Richardson 2013). This account emerges from his defense of the view that the efficient cause is simultaneous with its effect. He sees the opposite view—that the efficient cause is temporally prior to its effect—as another common error, which arises from consideration of ordinary events and processes, such as construction and reproduction: the builder building is temporally prior to the existence of the house; the father inseminating is temporally prior to the existence of the child. Avicenna argues that this account misreports the proper effects of ordinary efficient causes. The proper effect of the builder is building, not the existence of the house, and the proper effect of the father is insemination, not the existence of the child. Builders and fathers are true efficient causes of building and insemination, but they are only auxiliary efficient causes of the existence of buildings and children. So, builders and fathers are simultaneous with, not temporally prior to, their proper effects. Still, their actions make some contribution to the existence of buildings and children. This contribution is acknowledged through their designation as auxiliary efficient causes of the existence of buildings and children. The claim that builders and fathers are only auxiliary efficient causes of the existence of buildings and children invites us to investigate the true causes of their existence. According to Avicenna, the proximate efficient cause of the existence of sublunary substance is the Agent Intellect, while its ultimate efficient cause is God.