Institutio Oratoria

The act was not committed, and, if it was committed, it was not by me.

But if the defence say,

Prove that the man was killed,

the burden falls solely on the accuser, for the accused can say nothing more against the charge except perhaps in the way of casting certain suspicions, which he should throw out in the vaguest terms, since if you make one definite assertion, you will have to prove it or run the risk of losing your case. For when the question lies between our statement and that of our opponent, one or other will be regarded as true. Thus when the point on which we relied for our defence is overthrown, there is nothing left but the points that tell against us.