Businesses and products are started with assumptions…..

“assuming we can get 100 widgets for $5 each and sell all 100 for $10 we will make $500!.”

and adjusted for reality…..

“Turns out, the widgets cost $5.75, we had to buy 144 and were able to sell 113 of them. We made a profit of $302, what should we do going forward?”

No one can tell the future, but everyone can assume multiple outcomes are possible, decide on a margin of safety, and learn when to say no.