Full Idea
Wood, fire, water, flesh, iron, are things that I know, and only known because I perceive them by my senses; these are immediately perceived, and so are ideas; ideas cannot exist without the mind; their existence consists therefore in being perceived.
Clarification
'without the mind' here means 'outside the mind'
Gist of Idea
Perceptions are ideas, and ideas exist in the mind, so objects only exist in the mind
Source
George Berkeley (Three Dialogues of Hylas and Philonous [1713], III p.220)
Book Reference
Berkeley,George: 'The Principles of Human Knowledge etc.', ed/tr. Warnock,G.J. [Fontana 1962], p.220
A Reaction
This makes no distinction between an idea and its content. Berkeley fails to grasp the weird concept of intentionality. Trees aren't in my head, just because I think about them!