know for certain

Because none of them will ever know for certain what he may or may not have done.

"I know for certain that I want to stay involved in cycling because it is my passion. I am an expert in cycling and I believe that I can still add a lot of value to the sport going forward."

The commentary on this action, even from some liberal outlets, was curiously tentative: that newspaper endorsements don’t really matter, anyway, and “We can’t know for certain what went into these decisions.”

But without testing, it’s impossible to know for certain.

The storm’s impact is an estimation, as scientists don’t know for certain the effect of this geomagnetic storm until it arrives near two satellites that are 1 million miles from Earth, said Sean Dahl, service coordinator for the Space Weather Prediction Center.