We're assuming our original assumptions will work out to be true.
We've all heard the old adage "to assume is to make an "ass" our of "u" and "me."" The quote I heard above must double that effect! Certainly we sometimes have to deduce, from the data at our disposal, a position or conclusion. More often though, assumptions are used as weak shortcuts for the lazy or incompetent.
With all the time constraints we have on our day, how do we actually determine a proper course of action when called on to do so? The answer is simple. One word.
You've heard it before: "Failing to plan is planning to fail." If you don't plan at least weekly, with a start of the day review of your plan, you will, more than likely, be controlled by events and circumstances instead of controlling your own destiny. Any planning calendar ( I use a Franklin Covey planner) will work.
List your tasks and prioritize them. List all the known meetings and such. Block out time that is yours to work on your top priorities. Be sure to set aside time for the unexpected. This simple planning will establish a framework within which to work; to control your time. Plenty of unexpected things will come up, but you'll know where (and if) you can fit them in.
Without a plan, you'll be stuck "assuming" you know what's going on.