Once Baby has learned a scope, it knows that the corresponding pair of streams is correlated, but it doesn't know anything about the function or dependencies that produce the correlation. Base fluents represent these dependencies.
Baby builds a contingency table for each pair of token values that it observes within a scope.
If a statistic on the table exceeds a significance threshold, then Baby learns that these token values are not independent. Note that base fluents extend over several timesteps. Baby learned base fluents that correspond to objects in its environment, such as the red rattle, and also to persistent states, such as holding something plastic in its hand.