People have used a simple glass and some water to define certain kinds of people. Most motivational speakers or books describe an optimist as someone who finds a glass half-filled with water as half-full as opposed to a pessimist who finds it to be half-empty. There is no objection to the above definitions but they do not describe the complete set of optimists and pessimists.
Consider that a glass is partially filled with water (or any other drink you want). An optimist will see even a 10% filled glass as somewhat filled rather than somewhat empty. (Assuming the guy is not thirsty). On the other hand, a pessimist will see a 90% filled glass as somewhat empty. The genius is always the entrepreneur who, even after getting an empty glass, will see the glass as an opportunity to be filled and will promptly fill it.
Footnote – A person who can discern that a half-filled glass is both half-empty and half-full will surely be an engineer ;-).
