Some (like Heraclitus) would say "we can't step in the same river twice," as it's constantly changing. IOW, the identity of the river is never the same at any two points in time ... and that any grouping of river-states under a single model is just an abstraction that isn't *really* real. The thing that's real is the constant changing components, the water molecules.

The same could be said of one's own mind... as the physical states and thoughts are constantly changing. The unified illusion of persistence over time is an abstraction of our own observations of ourselves. In a sense persistent identity implies some sort of global unity and static nature.

There's an element of this thought in Buddhism. Buddhism really cuts to the heart of many assumptions about language, and truth (even rejecting two-valued logic in favor of fuzzy logic).

But what's radically different in Buddhist thought, though, is there's not as much emphasis on believing one single thing. A Buddhist might say there are two correct ways to look at the picture, in that in one sense a river is a real entity, and in another sense it is not. That both ways of seeing it form the complete picture.

Personally, I prefer a more round-about way of speaking about an entity like an abstract conceptual model ... perhaps some elements of Buddhist thought have worn off on me. A (Platonic?) statement like "everything that exists has a specific nature" seems like it's saying that the thing exists first before the properties. But IMO, it's only because we group similar perceptions under a common model ... like a river.