There would be a natural explanation for everything that might happen to him.

A black hole seems to be the only really natural explanation of the observations.

To others, however, a less natural explanation is more likely.

But then a simpler and more natural explanation came to my mind.

A cloud for which the astronomers could not agree on a natural explanation.

However, there are a number of natural explanations for this phenomenon.

Chambers' theory offered no natural explanation of why changes took place in the way they did.

If they are not present, then, as always, we must accept natural rather than unknown explanations.

He was still assuming in the back of his mind that all this would have some "natural" explanation.

But eventually somebody would realize that there was no natural or human explanation of what happened.