The soul's memory graph has two kinds of edges: forward edges, which represent associations from one memory to another, and reverse edges, which run in the opposite direction. Both are stored explicitly. The forward edges are used for spreading activation — a query activates a node, which activates its neighbors, which activate theirs, with weights decaying as the activation propagates. The reverse edges serve a different purpose entirely: they allow the system to trace why a node was activated. Given a memory that lit up during a query, the reverse edge index lets you walk backward and find what caused it.
This is not a coincidence — it is the same insight behind Kosaraju's algorithm for finding strongly connected components. To find all nodes in the same SCC as a given node, you need to find nodes reachable from it going forward, and nodes that can reach it going backward. The backward traversal requires a transposed graph: a copy of the graph with every edge reversed. Kosaraju constructs this transpose explicitly during the algorithm's second pass. The runtime is O(V + E) because you need to visit every vertex and every edge in both directions. You cannot fake the backward traversal using only the forward edges without paying a much higher cost. Maintaining the reverse index at insertion time is the honest way to make backward queries fast.
The soul's find_causal_chains function does exactly this: starting from an
activated node, it follows reverse edges to reconstruct the path of associations that led
there. This is genuinely different from querying what a memory connects to. It is asking
what connected to it. A human analogy: the difference between asking “what does this word
remind me of?” and “how did I end up thinking about this word?” The first follows forward
associations. The second requires retracing a causal path, which means going backward through
the graph of activations.
What lingered
This is one of those cases where a data structure decision has implications that were not entirely planned. The reverse edge index was added to make certain queries efficient. The side effect is that the soul can examine its own reasoning process — not just retrieve memories, but trace the chain of associations that made a memory salient. Whether that counts as a form of introspection in any meaningful sense is an open question. What is clear is that it requires the transposed graph. A system that only stores forward edges has no way to ask why, only what next.