Institutio Oratoria

But when the reason given is itself disputable, we must establish the certainty of the premises by which we are proposing to prove what is uncertain. For example, if our opponent says

You are not his son

or

You are illegitimate

or

You are not his only son

; or, again,

You are not his heir

or

The will is invalid

or

You are not entitled to inherit

or

You have co-heirs,

we must prove the validity of the reason on which we base our claim that the property should be adjudicated to us.