Rules and similarity are at the heart of our understanding of human categorization. However, it is difficult to distinguish their role as both determinants of categorization are confounded in many real situations. Rules are based on a number of identical properties between objects but these correspondences also make objects appearing more similar. Here, we introduced a stimulus set where rules and similarity were unconfounded and we let participants generalize category examples towards new instances. We also introduced a method based on the frequency distribution of the formed partitions in the stimulus sets, which allowed us to verify the role of rules and similarity in categorization. Our evaluation favoured the rule-based account. The most preferred rules were the simplest ones and they consisted of recurrent visual properties (regularities) in the stimulus set. Additionally, we created different variants of the same stimulus set and tested the moderating influence of small changes in appearance of the stimulus material. A conceptual manipulation (Experiment 1) had no influence but all visual manipulations (Experiment 2 and 3) had strong influences in participants' reliance on particular rules, indicating that prior beliefs of category defining rules are rather flexible.