Philosopher George Berkeley is born near Kilkenny in Ireland. Most famous for his doctrine of extreme idealism (as opposed to realism), George Berkeley will assert that material substances and things do not exist. Instead, what we consider to be objects like rocks and chairs are really just "ideas" and sensations which only exist in our minds rather than in any external reality.
Berkeley will base this conclusion on the argument that the qualities we perceive through our senses can only exist when perceived - as ideas in our minds. Because objects are nothing more than collections of such qualities, with no more fundamental "essence" behind them (also known as the "bundle theory"), then it follows that objects themselves are merely ideas in our minds, with no more fundamental "reality" behind them.