Detects existence and presence of entities and states.
What It Does
Boolean.Presence neurons encode existential claims — whether something exists, is present, or is occurring. They activate on 'there is', 'there are', 'exists', 'has', 'contains', and implicit existence constructions. They are distinct from Identity neurons: Presence asks 'does X exist?' while Identity neurons encode 'what is X?'
How It Behaves
Presence neurons show the strongest firing in early layers, where the model first encounters an existential construction and must decide how to represent it. They are among the most widely distributed Boolean sub-types — the question of existence is fundamental to almost every kind of reasoning. Their early-layer concentration means suppressing them has downstream effects across all later semantic processing.
Research Example
In Gemma 2B, Boolean.Presence neurons fire strongly on 'There are 192 countries in the world' and 'There is no cure for this disease' alike — they are sensitive to the existential structure, not the factual content. The difference between a hallucinated entity and a real one often shows up first as a difference in Presence neuron firing strength.