to be is to be perceived: “But besides all the endless variety of ideas or objects of knowledge, there is likewise something which knows or perceives them, and exercises divers operations, as willing, imagining, remembering about them. This perceiving, active being is what I call mind, spirit, soul, or myself. By which words I do not denote any one of my ideas, but a thing entirely distinct from them, wherein they exist, or, which is the same thing, whereby they are perceived; for the existence of an idea consists in being perceived.” -George Berkeley, The Principles of Human Knowledge (1710) G.N.A. Vesey, Body and Mind, editor, George Allen and Unwin Ltd., 1964. Image: Fermilab National Accelerator.

esse est percipi