What is a chair? Well, a chair is a certain thing over there... But how certain? The atoms are evaporating from it from time to time, dirt falls on it and gets dissolved in the paint; so to define a chair precisely, to say exactly which atoms are chair, and which atoms are air, or which atoms are dirt, or which atoms are paint that belongs to the chair, is impossible.
To an excellent approximation of perhaps one part in 10^10, the number of atoms in a chair does not change in a minute, and if we are not too precise we may idealize the chair as a definite thing.
Feynman's Lectures on Physics
