Philosophers are motivated to do research concerning pattern recognition because of wide range of its applications. One of the pathfi nders of research in that area was Satosi Watanabe, who has been frequently commented in the literature concerning this subject. The rule of decrease in entropy and the rule of simplicity are described in the context of pattern recognition. Although the concept of entropy had been initially used in the area of thermodynamics, it could be adopted also in the fi eld of pattern recognition. The concept of entropy should be then suitable transformed. A few of examples of the entropy concept application and the relationship between entropy and simplicity are discussed in the article. Simplicity considered by Watanabe should be treated mainly as polynomial curve simplicity, however the issue is described in the wider context.