Edward Weston's concept of equivalents was quite novel at the time. He used clouds for his initial series, and it's not difficult to see why. We can all find our own equivalents if we take the time to look at clouds long enough. But identifying a familiar shape in a cloud formation is just the beginning of understanding equivalents - the journey can open up an understanding of photography itself.
Weston's pupil, Minor White, probably encapsulated it best when he said, " One should not only photograph things for what they are, but for what else they are".