Conclusions are conditional

A mathematician, a biologist and a physicist
are sitting in a street café watching people
going in and coming out of the 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 people coming out of the house.

“The measurement wasn’t accurate,” says
the physicist.

“No, no,” says the biologist, “they have
reproduced.”

“I don’t think so,” says the mathematician.
“If exactly one person now enters the house,
then it will be empty again.”

This joke was posted on Profession jokes.