Suppose some mathematical creature from the moon were to reckon up the human body; he would at once see that the essential thing about it was that it was duplicate.

A man is two men, he on the right exactly resembling him on the left.
Having noted that there was an arm on the right and one on the left, a leg on the right and one on the left, he might go further and still find on each side the same number of fingers, the same number of toes, twin eyes, twin ears, twin nostrils, and even twin lobes of the brain.

At last he would take it as a law; and then, where he found a heart on one side, would deduce that there was another heart on the other.

And just then, where he most felt he was right, he would be wrong.
It is this silent swerving from accuracy by an inch that is the uncanny element in everything.

It seems a sort of secret treason in the universe.