There are about as many different kinds of existentialism as there are existentialists, making it difficult to extract a general character type. One basic definition (which nonetheless is not agreed on by everyone calling themselves an existentialist) is the belief that the meaning and value of your existence is determined by you alone, and likewise that you have the ability to influence the meaning and value only of your existence and no one or nothing else's. A frequently given example is a chair, the meaning of whose existence may seem quite natural or inherent--it exists to be sat on--but which many existentialists would deny: however much you may sit on the chair, so the argument goes, its "meaning" or "essence" is something you impose on it, not something that inheres in it. From the chair's point of view, so to speak, you are a non-factor.
Different existentialists react differently to such a belief. Some find it to be cause for great pessimism, since the implication is that you are powerless to influence the meaning and value of anything or anyone outside yourself. Others find it to be cause for great optimism, since the implication is that your
meaning and value is something no one can take away from you.