Relativity.

If all the flowers were roses,

A rose would not be rare;

If all the days were cloudless,

We’d call not any fair.

If all the books were bibles,

But few would learn to read;

If all the world were Christians,

There’d be no better creed.

If everything were scarlet,

There’d be no black or white;

If there were nothing evil,

There’d be no wrong or right.

If every zone were frigid,

There’d be no milder clime;

If everybody pilfered,

To steal would be no crime.

If people did not differ

In sentiment and thought,

Mankind would be a unit,

And life would be for naught.