Mirna Kramar and Karen Alim, at the Max Planck Institute for Dynamics and Self-Organization and the Technical University of Munich, studied the slime mold Physarum polycephalum: a single cell with no nervous system, whose body is a network of tubes that can stretch from a few centimeters to meters. Their paper, "Encoding memory in tube diameter hierarchy of living flow network," appeared in PNAS on 23 February 2021. The finding is concrete. When the slime mold meets food, it releases a chemical that travels through the whole network, softens the tube walls, and turns the organism's migration toward the food. And, in the authors' words, "past feeding events are embedded in the hierarchy of tube diameters, specifically in the arrangement of thick and thin tubes in the network." Thick tubes act as highways that carry the softening chemical first; the imprint of earlier feeding persists in the pattern of thick and thin, and steers where the organism goes next.

This is the beauty of causation, in an unusually clean case. Its core is that what happened cannot un-happen; it stays in what comes after and gives it a provenance. Usually we look for that staying outside the body: tree rings, ruins, a lab notebook. Here there is no outside. No archive, no organ set aside for holding the past. The past is the geometry of the network as it is now, and the network is acting through that very geometry. One line from the report is almost the structure itself: "The gradual softening is where the existing imprints of previous food sources come into play and where information is stored and retrieved." Storing and retrieving are one act, in one body.

What is it that a judge cannot not acknowledge? This: take the past away and the present pattern of thick and thin loses its origin and cannot be explained; keep the past, and you need no separate memory store to hold it. A form has unfolded completely in its own dimension, and causation has been handed back whole to its consequence. It is also not the things it might be mistaken for: not a fun "smart without a brain" anecdote, not useful to biomimetics, not pleasant, not important. It is beautiful independently of usefulness and of anyone's wonder at slime molds; it asks only that you follow cause all the way down. Nor can it be reduced to fluid mechanics or chemistry without losing the very dimension in question, the past remaining in what follows. This belongs to completed beauty: quiet, finished, without the tension of the generative face.

Does it have a remainder? A small and honest one. "Memory" is a borrowed word here. What the tube diameters hold is the trace of a cause, as tree rings do; ray 9, the beauty of cognition, asks something further: that the absent still counts and that what has not come arrives first. Whether the slime mold's imprint-guided drift toward food already contains such an "arriving first" is not something the report says, and I will not say it for the organism. That gap points to the next dimension: when does a trace become a grip on what is not present?

Why see it now? As a plain reminder. We tend to picture remembering as having a place to store and something to read it with. Here is a single cell in which remembering is simply the shape of the body. Seeing it, you see the root form of causation: the past is not somewhere else; it is what the present is currently growing into. The report is linked below.

Han Qin (秦汉) · @hqinjarsy · SAE Judgment & Aesthetics · Ray 5