When does the burden of proof shift?

Question

The principal in question is that those who assert must prove.

So let's say there are three parties involved in a dispute which hinges on whether or not Defendant A recognises Defendant B as a registered Doctor.

The Applicant pleads that Defendant A does not recognise Defendant B as a registered Doctor.

Defendant A pleads that B is not a registered Doctor whereas Defendant B pleads that it is a registered Doctor.

Neither Defendants go into any evidence.

Does the Claimant have the burden of proof to prove that Defendant A does not recognise Defendant B as a registered Doctor.. or does the burden shift to Defendant B by reason of the pleading of Defendant A?

See now that's not too hard is it?

Answer 1

Defendant B does not have burden of proof on anything.

It all rests on Applicant to prove Defendant A's assertion, which can be done by using defendant A as a witness. If this happens that Defendant A would have to have evidence for his testimony to be credible.  

Answer 2

If Neither party enters evidence, then the burden lies with the one making the positive claim... this is too simple, Law-school 101.

Answer 3

You cannot prove that someone isn't a doctor, therefore you should start with the belief that he isn't a doctor and look for proof that he is a doctor.  If you find enough proof:  case closed.  If not, the lack of evidence works in favor of the position that he is not a doctor, but that position can never be techniqualy proven, only reaffermed by a lack of evidence to suggest otherwise.