In 1689 Locke had argued that the mind is a blank sheet and all knowledge comes from experience — while also assuming that objects have a 'material substratum' existing independently of our experience. George Berkeley, a young Irish clergyman, showed that this assumption contradicts empiricism's own principle: we never perceive matter, only colour, hardness, sound and smell. To assert the existence of something never perceived is to advance a claim that cannot be drawn from experience.

The result is his famous formula: *esse est percipi*, to be is to be perceived. Berkeley presents this not as denying the material world but as correcting its misdescription: the tree is real, but its reality is a bundle of perceived qualities, not an unknowable substance behind them. To the question of whether the tree continues to exist when nobody is looking, his answer is theological: God perceives everything continuously, so the world's continuity is secured.

His position struck most contemporaries as absurd — Samuel Johnson kicking a stone and saying 'I refute it thus' is the emblem of that reaction, though the argument never claimed the kicked stone was anything other than its perceived hardness. The real effect appears in Hume: applying the same empiricist principle to mind and causation as well as to matter, Hume arrives at scepticism, and with it the question that woke Kant from his 'dogmatic slumber'. The chain Locke–Berkeley–Hume is the backbone of modern epistemology; without the middle link the next step cannot be understood.