The assumed proposition “Some stones are humans” identifies an object as both a stone and a human. But “No human is a stone” then excludes that same object from the class of stones.
Reasoning:
1. Some stones are humans — hypothetical negation of the converse.
2. No humans are stones — true original.
3. Therefore some stones are not stones — denying of the very thing already asserted.
This is the impossibility of denying a thing of itself. Therefore the initial assumption fails and the universal-negative converse stands.