Fuzzy Set Theory, or Fuzzy Logic, allows to program computers to 'make decisions' based on imprecise data. Fuzzy Logic reflects how people think, it models our sense of words, our decision making, our intuition. It allows us to operate with linguistic variables such as 'delayed' or 'late' payment, ‘average’ or 'very important' customer, ‘similar’ names. Fuzzy Set Theory makes it possible to program linguistic rules such as 'IF vendor is Not Important AND voucher amount is Small THEN pay Late'.