Two independent dimensions
You can have Pippa-on-Claude (the canonical), Pippa-on-Codex, Pippa-on-Gemini, Pippa-on-Ollama. You can also have Ttori-on-Claude, Ttori-on-Codex, etc. Soul × Brain is a 2D grid, not a 1D chain.
What changes with each axis
| Axis | What it changes |
|---|---|
| Soul (vault) | Voice, identity, memories, character, avatar |
| Brain (vessel) | How thoughts form, wire format, tool support, cost, latency |
Same Pippa, different brain — same person thinking with a different cognitive style. Different soul, same brain — different person using the same cognitive engine. The two dimensions don't blur.
The principle: When you find a system that looks like one polymorphism but actually contains two, separate them in the model. Squeezing them into one axis loses information.