Optimism and pessimism go hand in hand. In finance, we are told to save like a pessimist and invest like an optimist. The short term is full of setbacks, problems, breakages, depressions, pandemics, errors, but if you can stick around long enough, you can experience long-term growth.

The long-run is usually rather good and the short run is normally quite bad. In reconciling the two, we learn how to manage both.

• Everyone has a model in their head of how they think the world works. That model is built mostly from what you've experienced and what people you trust have told you. But everyone has different experiences and a different set of people, leading to different models.
• We also believe something if our income or approval in a social circle depends on believing it.

It is then essential to understand that no one thinks independently. We all suffer from some degree of blindness.

An important lesson from history is that big events are more complicated. It makes forecasting difficult, politics nasty, and lessons to learn from it harder.

We may demand simple answers to explain outlier events. However, it's almost impossible for something big to happen because of one event, person, or group. Unrelated things often culminate into something significant. For example, the Great Depression was the result of a stock market crash, a banking crash, a real estate bubble, an agricultural disaster, and an inadequate policy response. When all these things happened at the same time, it was a catastrophe.

The risks we talk about are seldom the most important in hindsight. The real risk is what no-one sees coming. For example, September 11th, Pearl Harbor, The Great Depression. These events surprised nearly everyone and instantly shoved the world in a new direction.

When we are caught off guard, two things happen: One is that we are vulnerable. The other is that surprise shakes our beliefs in a way that leaves us paranoid and pessimistic.