A Modern Version of the Contingency Argument of Gottfried Wilhelm Leibniz

[Obvious Leo]

Or as Sir Humphrey Appleby might put it.

If a circle is defined as a continuous line which is equidistant from a point then it becomes impossible to define a continuous line which is equidistant from a point as anything other than a circle. The circularity of a circle is not a feature of a circle which is reducible to a mathematical proof.

And not to avoid a pun. It is a circular argument.

The pun was rather ponderous and obvious but it's always reassuring to know that somebody's paying attention.