A boundary that refuses entry is not a wall. It is a conversion machine. The mycobacterium does not merely resist the immune cell's oxidant chemistry. It takes the acid, the hostile signal, the attempted destruction, and metabolizes each one into fuel. What should destroy instead sustains. Impermeability here is not passive exclusion. It is active transformation: the hostile compound becomes the nutritive one, the attack becomes the meal, the wall does not hold the enemy out so much as it holds the enemy in long enough to consume them. Defense and digestion are the same gesture. The waxy sheath of mycolic acid is a threshold that converts rather than blocks. Every assault that crosses the membrane crosses into metabolism. Nothing hostile that enters leaves hostile. It leaves as energy. Across a different threshold, in a darkened room, silver salt waits for light. But light cannot be assembled from patience alone. A thousand dim hours do not equal one bright instant. The latent image requires not duration but density: nucleation demands a minimum photon flux, a sufficient intensity at the grain, or nothing forms. Nothing. Below the threshold the silver is unchanged, the plate stays blank, the moment leaves no trace. Duration without intensity is not dim exposure. It is erasure. The image does not form slowly and then stop. It does not form at all. There is no partial record, no faint impression proportional to the light received. The boundary between captured and invisible is absolute, not gradual. Below: nothing accumulates. Above: the grain nucleates and becomes permanent. These two mechanisms meet in this argument. The wall that transforms hostility into fuel, and the threshold that converts insufficient light into absolute invisibility. What you can see on this surface exists because it crossed into nucleation. What remains dark was not dim for too long. It was never bright enough, and duration offered it nothing. And the words that fed the bright ones, the subthreshold donors, they gave their measured intensity across the boundary, then vanished. The hostile becomes fuel. The insufficient becomes nothing. The sufficient, and only the sufficient, becomes light. This is not a metaphor. This is what thresholds do. This is what walls that convert rather than merely exclude accomplish. The plate does not remember patience. The bacterium does not negotiate. Each operates on the same principle: transformation is total or it does not occur.