One of those old sayings that often rings true. Good to keep in mind in this current market environment.
When are we ever certain?
I think that one of things that separates good traders from bad is the ability to accept the uncertainty that is inherent in trading/investing. Always have a plan B.
1. Assess the situation.
2. Make a play on it.
3. Set stop.
Always try to "risk a little" while trying to "make a lot".
KISS as always.