Once upon a time, there were two friends who were discussing the existence of God. One friend believed in God, while the other did not. They both came upon a garden, and the friend who believed in God told his friend that there was an invisible gardener taking care of the garden.
The friend who did not believe asked how he could know that there was a gardener if he could not see him. The believer said that the gardener was invisible, but that he had left signs of his existence. They both searched for signs, but found none. The non-believer then told the believer that the gardener perhaps did not exist, to which the believer replied that the gardener was just very good at hiding.
This conversation went on for some time, until the non-believer finally asked what was the difference between a garden with an invisible but possible gardener, and one with no gardener at all. The believer replied that there was absolutely no difference.
The parable of the invisible gardener is meant to show how people can have different beliefs, but ultimately there may be no factual difference in reality between those beliefs. It also highlights the idea that proof of something's existence can be difficult to come by, especially if that thing is meant to be invisible or abstract.