If pessimism is despair, optimism is cowardice and stupidity. Is there any need to choose between them?
As a world view, Darwinism cannot of course be refuted, since Faith is, always has been, and always will be, stronger than facts.
The purest expression of the doctrine of Liberalism was probably that of Benjamin Constant.
Pessimism only describes an attitude, and not facts, and hence is entirely subjective.
Rationalism, which is the feeling that everything is subject to and completely explicable by Reason, consequently rejects everything not visible and calculable.