Something that decides whether a percept carries features or only a location.

Mirrors tbp/monty/frameworks/models/sensor_modules.py::PerceptFilter, the protocol a sensor module's filter answers. CameraSM passes every percept through one: the default passes everything on, and FeatureChangeFilter sends a point to a learning module only when something about it changed enough to be worth learning.

A filter is a value here, so what it remembers travels beside it rather than inside it: the callback takes the filter, the percept and that memory, and answers both the percept and the memory's next version.