The discussion here is interesting. In general, I don't think either optimism or pessimism are the issue. Letting your feelings overwhelm data is the issue. It

The discussion here is interesting. In general, I don't think either optimism or pessimism are the issue. Letting your feelings overwhelm data is the issue. It's fine to be optimistic so long as you can realistically assess when the idea isn't panning out. Same about pessimism -- it's fine, so long as you are willing to investigate and get some data. There are lots of bad ideas, so undue optimism needs pruning. There are also lots of good ideas we haven't tried yet -- that is the evidence of history. The big problem I see in life -- including my own life -- is attachment to positions when the data is not in accord with that attachment. In science, it's why you often have to wait for a new generation of scientists who can fully integrate newer results (data) into their thinking. In life, it's why people like me get stuck in the past: it's the lens we understand.