To be is to be perceived (Esse est percipi)." Or, "If a tree falls in the forest and no one is there to hear it, does it make a sound?

The same principles which at first view lead to skepticism, pursued to a certain point, bring men back to common sense.

But the velocities of the velocities - the second, third, fourth, and fifth velocities, etc. - exceed, if I mistake not, all human understanding.

I had rather be an oyster than a man, the most stupid and senseless of animals.

Doth the Reality of sensible things consist in being perceived? or, is it something distinct from their being perceived, and that bears no relation to the mind?

But, say you, surely there is nothing easier than for me to imagine trees, for instance, in a park [. . .] and nobody by to perceive them. [...] The objects of sense exist only when they are perceived; the trees therefore are in the garden [. . .] no longer than while there is somebody by to perceive them.

I might as well doubt of my own being, as of the being of those things I actually see and feel.