A physicist, a chemist, and an economist are stranded on a desert island. One can only imagine what sort of play date went awry to land them there. Anyway, they’re hungry. Like, desert island hungry. And then a can of soup washes ashore. Progresso Reduced Sodium Chicken Noodle, let’s say. Which is perfect, because the physicist can’t have much salt, and the chemist doesn’t eat red meat.
But, famished as they are, our three professionals have no way to open the can. So they put their brains to the problem. The physicist says “We could drop it from the top of that tree over there until it breaks open.” And the chemist says “We could build a fire and sit the can in the flames until it bursts open.”
Those two squabble a bit, until the economist says “No, no, no. Come on, guys, you’d lose most of the soup. Let’s just assume a can opener.”
You can’t just assume a can opener and have it appear, obviously. But economists regularly rely on analogous assumptions of conditions that don’t actually exist in the real world. This is one of the fundamental ways in which economics differs from the hard sciences, even though economists often seek to present their views with the certainty of, say, a physicist or chemist.