Through numbers of studies on the formation of equivalence relations and causal induction, it is known that human beings tend to consider conditional statements ``if p then q" as biconditional statements ``if and only if p then q": we call the tendency to perceive ``if p then q" as ``if q then p" the ``symmetry bias". On the other hand, many studies on children's word learning have pointed out that children tend to expect each object has only one label. This is so-called the ``mutual exclusivity bias". This bias implies