If we take a fertilized hen's egg, so a chick will hatch out, if we incubate the egg at the right temperature. Everything what the chick requires to its development is contained inherently in the egg. Let us hold the egg too cold or heat it too high, so the life in the egg will die out. We can use the egg at most still for nutritional purposes. A chick never will hatch.
All birds hatch out, if they are properly incubated.
And this happens without any help from man. Nature contains all chemical matter, which is required for the philosophical egg. However in nature this secret egg nowhere is found in a perfect state. Because the basis to the philosophical work cannot be found in perfect composition, a conscious being must produce this basis artificially. Therefore alchemy is called an art.