Le Morte D'Arthur
How we cite our quotes:
"This nyght ye shalle lye with Igrayne in the castel of Tyntagyll; and ye shalle be lyke the duke her husband." (5.8-9)
Arthur is conceived when Uther swaps identities with Igrayne's husband, Gorlois – the duke. Of course this disguise makes Arthur's right to the throne all the more confusing. Who is going to believe that this illegitimate child with a murky history can legitimately be King of England? Not the alliance of Northern Kings, that much is sure.
"Now," said Sir Ector to Arthur, "I understonde that ye must be kynge of this land." "Wherefore I?" sayd Arthur, "And for what cause?" "Sire," saide Ector, "for God will have hit soo, for ther shold never man have drawen oute this swerde by he that shal be rightwys kyng of this land." (9.25-29)
Here, Arthur's pulling the stone confirms one part of his identity: he is King of England. But that's all it does. It does not explain why Arthur has the right to be king, which is a big problem for the other noblemen, since his identity as Uther's son is still unknown, even to Arthur himself.
"Also I know what thou arte, and who was thy fadir, and of whom thou were begotyn: for Kynge Uther was they fadir and begate the on Igrayne." (31.25-27)
Merlin's revelation to Arthur in the likeness of a child begins Arthur's process of discovery surrounding his true identity. After this, Arthur insists on having the information he learns from the "child" verified by his foster-father and by his mother, Igrayne. Perhaps Merlin decided to appear in this disguise so that Arthur would be forced to verify his information, which will prove his parentage beyond a shadow of a doubt.