We demonstrate that plasmodium of Physarum polycephalum perceives an ambiguous stimulus made from oatmeal and KCl as it is either attractant or repellent. While there is no intermediate behavior of being attracted and avoiding a stimulus, each plasmodium makes a decision whether a given stimulus is either attractant or repellent evenly. We propose a model explaining why the plasmodium either covers a stimulus perfectly or avoids a stimulus without entering a stimulus. The model is based on a rough set analysis. We propose that the plasmodium has two perception models with respect to attractant and repellent. These two models conflict with each other. Due to the composition of two models, the effect of one of the models can be evoked and the effect of another model is suppressed.