esse est percipi (aut percipere) — to be is to be perceived (or to perceive)
(George Berkeley (12 March 1685 – 14 January 1753), also known as Bishop Berkeley (Bishop of Cloyne), was an Anglo-Irish philosopher)
The theory of "immaterialism" developed by George Berekly (later referred to as "subjective idealism" and "idealism").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.
According to this argument, all of the qualities attributed to objects are sense qualities. Thus, hardness is the sensing of a resistance to a striking action, and heaviness is a sensation of muscular effort when holding the object in one’s hand, just as blueness is a quality of visual experience. But these qualities exist only while they are being perceived by some subject or spirit equipped with sense organs. This is a rejection of the idea that sense perceptions are caused by material substances.