Fig. 1.

Illustrative plankton specimens. The plankton in the left half of the figure belong to species a, and those in the right half of the figure belong to species b. Note that only the eye (which can be yellow or black) and claw (which can be dark or light green) vary across the specimens. (See Figs. S1–S3 in the Supplemental Material available online for the actual stimuli; these examples have been altered to make the differences between features clearer in print.) Because of the probabilistic distribution of the features within each species, most specimens cannot be identified as species a or species b with certainty (i.e., the combination of black eye and light claw occurs in both categories). Assuming the observed specimens match underlying probabilities, the probabilities are as follows: P(species a|yellow eye) = 1, P(species b|black eye) = 8/13, P(species a|light-green claw) = 7/8, and P(species b|dark-green claw) = 7/8. Information gain, impact, and probability gain agree that the claw is more useful for categorizing a random specimen than the eye is, but only the eye offers the possibility of certainty.