Decay theory
Decay theory is the claim that memory traces fade or weaken over time if they are not actively rehearsed or used. It is the default folk explanation of forgetting: memories, like unused muscles, atrophy from disuse.
This explanation is wrong, and the wrongness is instructive. Memory traces do not decay because they are unused; they decay because the system that maintains them is dynamical, and dynamical systems do not preserve states in the absence of active maintenance. The "decay" of a memory trace is not the erosion of a stored object. It is the relaxation of a neural attractor to its baseline state — the system's return to equilibrium when the perturbation that created the attractor is removed.
In this view, forgetting is not a failure of storage but a feature of design. A brain that preserved every transient activation pattern would be a brain that could not adapt. Decay is the price of plasticity. The relevant question is not why memories decay but why some memories are maintained against decay while others are allowed to relax. The answer is not rehearsal; it is the depth of the attractor, the strength of the synaptic modification, and the coupling between the memory system and the motivational systems that signal relevance.
Decay theory survives not because it is true but because it is the only forgetting mechanism compatible with the storage metaphor. Once you abandon the metaphor, decay becomes a tautology: memories "decay" because they are not maintained. Everything that is not maintained ceases. The explanation is not in the memory but in the maintenance.