Irreversible cognitive transitions mark the hierarchy of symbol grounding

How symbols acquire meaning from sensory experience has remained experimentally intractable because the grounding process cannot be observed in any system that has already completed it. Here we present a minimal encoder–decoder system in which the complete grounding trajectory unfolds under continuous measurement. Five grounding criteria form a strict hierarchy marked by irreversible transitions. Ungrounded models match in-distribution performance but achieve no analogical transfer. 430 independent runs across 16 pipeline configurations, scales spanning 343K to 944M parameters.