A Treatise Concerning the Principles of Human Knowledge is a work by Empiricist philosopher George Berkeley. The world which caused the ideas one has within one's mind, Berkeley sought to prove that the outside world was also composed solely of ideas. Berkeley did this by suggesting that "Ideas can only resemble Ideas" - the mental ideas that we possessed could only resemble other ideas (not physical objects) and thus the external world consisted not of physical form, but rather of ideas. This world was given logic and regularity by some other force, which Berkeley concluded was God. 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").