power of positive thinking

there has been a lot of talk about optimism and pessimism and the importance of optimism in life. even studies on cancer has shown optimism helps in treatment process. the often quoted 'half full- half empty' glass has often been really puzzling. only if one realises that the glass is half empty, it wil be filled. or it wil remain the same because it is half FULL! recently read something- half empty, half full, or a glass that need to be washed. make sense :)