From Neal Stephenson's The System of the World, the concluding volume of The Baroque Cycle, a great read:

...I saw one or two smoke-rings about the size of a man's hat, propagating across the room, and retaining their shape and vis viva for extraordinary distance. These rings are unlike water-waves, which consist of different water at different times, for smoke rings propagate through clear air, proving that they indeed carry their own substance with them, neither diluting it with, nor dispersing it into, the surrounding atmosphere. And yet there is nothing special about the smoke as such -- it is the same smoke that hangs over battlefields in shapeless clouds. The identity of the smoke ring would appear to consist, not in the stuff of which it is made, for that is commonplace and indifferent, but rather in a particular set of relationships that is brought into being among its parts. It is this pattern of relationships that coheres in space and persists in time and endows the smoke-ring with an identity. Perhaps some similar observation might be made about other entities that we observe, and credit with uniqueness and identity, including even human beings. For the stuff of which we are made is just the common stuff of the world, viz. ordinary gross matter, so that a materialist might say, we are no different from rocks; and yet our matter is imbued with some organizing principle that endows us with identities, so that I may send a letter to Daniel Waterhouse in London in full confidence that, like a smoke ring traversing a battle-field, he has traveled a great distance, and persisted for a long time, and yet is still the same man." (a fictional Gottfried Wilhelm von Leibniz writing to Daniel Waterhouse; p54)