The weather and the many other things change on a whim. Human nature itself is fickle. The stock market fluctuates. The value of our possessions depreciate. Our future is uncertain. It's all so unpredictable. Notions come and go as with varying 'elements' of fashion which clutter the fundamentals of our existence. With a stone however, you know where you stand. The elements of the mineral world have combined long ago. They have made up their minds, and decided to stay with it, and more importantly, stick it out for eternity or at least until heaven and earth pass away. This is something you can count on. A physical stone is something you can actually grasp and 'put your finger on'.
Look out over the endless fields of choice, doubt and misunderstanding. A dry stone wall passes through these things, onward over the landscape in an ordered march of solid purpose. In a wall there are few areas of indecision or preponderance. Everything is reduced to a simple number of considerations. Nothing is taken for granted or over emphasized. You work with what you have, and although there may be a lot of stones and a lot of time, there doesn't really have to be a dazzling variety of anything else.