Optimism is the madness of insisting all is well when we are miserable.These words are reported to have come from Voltaire, the acclaimed French writer and philosopher. I am neither a Frenchman or a philosopher but I would like to put forward the suggestion that Optimism is actually the only way! When backed by concrete evidence it is the only rational belief to have, particularly so when it comes to investing your money over the long term.This piece aims to convey the rationale for such a belief whether you are savings ?100 per month into a savings or pension plan, or have ?10 million invested in a well diversified equity portfolio. This is not a debate about the virtues of owning equities over commodities, or for indeed property. Developing and nurturing this belief is the single most important attribute to have in achieving long term investment success. We can't predict the future values of equities, nobody can. The future is always uncertain, however it is rational and human to base our expectations and beliefs on what has gone before.