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.
At least, in terms of long/short positions intended for longer term (week or more) holdings, and without nose-to-screen monitoring. An ADX of anyone's parameters is well below 25 -- "Woof!" (On the other hand, a VIX above 20?? == $$$! for the option crowd.)
1. Assess the situation. 2. Make a play on it. 3. Set stop. Always attempt to "risk a little" while trying to "make a lot". KISS as always.