The body of science is not, as it is sometimes thought, a huge coherent mass of facts, neatly arranged in sequence, each one attached to the next by a logical string. In truth, whenever we discover a new fact it involves the elimination of old ones. We are always, as it turns out, fundamentally in error.
If you bounce a tennis ball against a wall it will come back to you the same way every time. But if you shift the wall a few degrees it will come back another way.