“Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house.”Henri Poincaré · Science and Hypothesis · 1902
Geometry is a practical convention, not an absolute truth, chosen to simplify calculations.
Poincaré showed you can describe the same world using different geometries, just like you can map the same city on a flat sheet or on a globe. We pick ordinary geometry not because it is the only true one, but because it makes calculations simplest. The choice matters, even if invisible.
When you choose a subway map, it does not matter if the lines are straight or curved, only if it helps you reach your destination. Similarly, Poincaré said that the geometry we use is not an absolute truth, but a practical tool. We could have chosen a different geometry, but we picked the one that makes calculations simplest. For example, when you measure the distance between two points on a map, you do not check if the rules of geometry are correct, only if they work. This invisible choice shapes everything we know about space.
Why it matters This idea helped Einstein, a few years later, to think beyond ordinary geometry.