Thiel's concept of the definite optimist. It is not enough to be an optimist. You need to think about the future in a definite manner. That does not mean that everything you plan will work out. It is easier said than done. But being a definite optimist is the first step to be in control of your destiny.
Developing optimism and resiliency isn't about feigning a facsimile of terminal perkiness and blind optimism. It's quite the opposite. It's having the courage to see and meet life exactly as it is. It's all about choosing to learn how to respond to each adversity with confidence, hope and renewed vigor.