I enjoy building sand castles at the beach. My standard model is the hole surrounded by walls. First, I dig a hole in the sand deep enough to reach the water table. This is the limit to how deep the hole can be because once the water table has been reached the bottom of the hole begins to get wet. If I continue digging the sides of the hole will collapse into the bottom. At this point the hole will only get wider and wider but not deeper. The sand removed from the hole is then used to build a wall around the hole to prevent the ocean waves from pouring into the hole.
Eventually, the ocean waves breach the walls. When this happens the hole fills up with water and sand. It is amazing how quickly a single wave can erase hours and hours of work. Sometimes the entire castle is completely erased. Usually there is some smooth reflection of the fruits of my labor. At this point I can decide to give up or to dig out the hole once more. Usually I am good for three or four generations before I give up.
When the tide goes out the building is easy because the waves become less and less threatening over time. When the tide comes in it becomes harder and harder to keep the waves from overrunning the perimeter and filling the hole and eventually the ocean will always win.
I have never built a sand castle that could withstand the ocean unless it is built so high up the beach that the ocean could not reach it. But even under those circumstances it will either erode for other reasons or some kind of blue moon tide will wash it away eventually. The paradox is that once the castle is safe from the waves it is no longer as fun to build. There is something about the threat of annihilation that makes the castle worth building in the first place.
So what is the purpose of this endeavor, to build something that will only be certainly destroyed? There is an interesting life parallel. As humans we struggle to achieve something despite the fact that death is inevitable and whatever we do achieve during this life will be forgotten over time. We might delude ourselves to think that our legacy will be passed along to our children. But even our descendants will have no real sense of our existence a mere 100 years down the line. Eventually the Earth itself and all humanity will be consumed by the sun (assuming interstellar colonization never comes to pass). So really any concern about progeny beyond our great grandchildren is a vanity and a chasing after wind. In the same way concern for the longevity of a sand castle is a vanity and a chasing after wind.
I imagine a Buddhist would agree with the notion that building a sand castle is folly. He might say that it would be better to not build the sand castle in the first place. The ocean can never destroy that which was never built in the first place. By contrast, I imagine a Christian might encourage me to build a sand castle anyway despite the knowledge that it will be destroyed. For it is in striving to be better that I refine my soul. Perhaps an atheist would say it does not matter whether I build the sand castle or not so I might as well enjoy the time I have.
It is interesting to note that when fighting the ocean the walls of the sand castle become far more important than anything they were originally built to protect. Often there is never anything within the walls except a hole to the water table. The walls are built to resist the ocean (a force that they can never hope to withstand) to protect nothing and when the sand castle is eventually erased everything is back to its original state. There is something poetic about that.
To strive and achieve is somehow making the most of our mortal existence. But to grasp at leaving something material behind after death seems like a fools errand and an impossibility to boot. Perhaps building a sand castle is being grateful for the opportunity to build a sand castle. Or maybe building a sand castle is to immerse one’s self in the action of building a sand castle. Or maybe building a sand castle is to acknowledge the ultimate equality of the millions of grains of sand that are used to construct the sand castle. Together they make a sand castle and individually they are nothing but sand.