The key property is heterogeneity among thought trials. The advantage of blind-variation, after which thought trials are modeled, is that the process can be "smarter" than the people who run it. If one thought trial has a minimal effect on the generation of the next thought trial, then a broader range of possibilities is tried. Given the tendency of humans to exhibit grooved, habituated, redundant thinking (Stein bruner, 1974), this requirement of good theorizing is among the most difficult to meet regularly.