For a given assertion, the burden of proof returns classically to that which affirms it, but in certain cases the burden of proof returns to that which refutes the validity of it. One speaks then about inversion of the burden of proof .

This inversion of the burden of proof can be justified when the assertion refers to an indetermination or an uncertainty. ex: to affirm that the existence of the Father Christmas is unspecified. Such an assertion cannot be proven, whereas its refutation can the being. The refutation is it even an assertion. It is much more restrictive (strong) that the first and of this fact more easily testable.

The burden of proof is thus determined by the facility with which it is possible to prove an assertion. If the assertions which clash are on the one hand that the Father Christmas exists, and on the other hand that there does not exist. The burden of proof returns to that which affirms that there exists because this assertion is easier to show than its reverse. It is indeed easier to show than something exists, than something does not exist.

Remarks relating to the burden of proof and its inversion

• the absence of proof of an assertion and of its exact reverses shows at most that the validity of the assertion is unspecified. Thus, to affirm that a rabbit leg brings a supplement of happiness, with the reason that the reverse shown forever is fallacious. The validity of the assertion remains unspecified.

• So that an assertion is established like true, it is necessary that it is proven true or that its exact reverses is shown distorts (reductio ad absurdum). A frequent error is not to consider the exact opposite one.