Getting into a muddle by numbers

A MATHEMATICIAN, a biologist and a physicist are sitting in a street cafe watching people going in and coming out of a house on the other side of the street.

First they see two people going into the house. Time passes. After a while they notice three persons coming out of the house.

The physicist pronounces: "The measurement wasn't accurate."

The biologist concludes: "They have reproduced."

The mathematician declares: "If now exactly one person enters the house then it will be empty again."