In An Essay Towards a New Theory of Vision George Berkeley weaves a theory that depends on God's existence, and is shockingly difficult to refute. The problems he poses are immensely difficult, and still being answered. George Berkeley was an Anglo-Irish philosopher whose primary achievement was the advancement of a theory he called "immaterialism". This theory denies the existence of material substance and instead contends that familiar objects like tables and chairs are only ideas in the minds of perceivers, and as a result cannot exist without being perceived. Thus, as Berkeley famously put it, for physical objects "esse est percipi" ("to be is to be perceived").