Destiny Times Three by Fritz Leiber

And many a Knot unraveled by the Road But not the Master-knot of Human Fate. The Rubaiyat

Our quest for our Probability Engine and its talismans has occupied many major units even of our own time. We have prosecuted it with diligence, because we were aware of the dangers that might arise if the engine were misused. We built several similar engines to aid us in the search, but it turned out that the catastrophe in our cosmos which swept away the engine and cast one of the talismans up on your time-stream and planet, was of an unknown sort, making the route of the talisman an untraceably random one. We would have attempted a canvass of reality, except that a canvass of an infinitude of infinitudes is impossible. Now our quest is at an end.