Something can't be both true and false at the same time, it's common knowledge, but when the tables turn and reality changes from it's deep core not even hope can hide the truth, and maybe it's for the best, who knows?
Contrary to popular belief, Arthur isn't as oblivious as he seems, not anymore at least. He can't even remember when it was that he saw Merln's magic for the first time, or what he was doing with it. He knew that Merlin had magic and that magic was evil, therefore Merlin must be evil, he thought, but he immediately corrected his own mistake, Merlin wasn't evil and he knew that perfectly. The following days he followed Merlin everywhere he went and nothing evil was done by the warlock so he arrived to the conclusion that magic is both evil and it's not, depending on the person wielding it. After that he waited, he waited for Merlin to tell him, to trust him. When the king was on his deathbed and he went looking for Dragoon he knew well enough that Merlin was the old man, but when Uther died he was mad, blinded by hatred until he overheard Gaius and Merlin talking about the amulet Morgana had placed around Uther's neck. The newly made king hadn't felt as guilty ever, because of everything he'd told his best friend about magic.
He waited, and waited and kept waiting, but Merlin never told Arthur his secret and the king was growing impatient. He accepted the fact that his friend would never trust him enough to take the first step so he took it himself.
One morning as any other, Merlin went into the king's chambers, but instead of finding him fast asleep, the king was wide awake and already dressed. "We're going hunting" said Arthur fully expecting the loud moan from Merlin that followed his statement.
Half an hour later they were both on their way into the woods. The day was incident free, and as night fell Arthur had already hunt enough rabbits for the whole court of Camelot, so they stopped the horses and made camp. The previous day had rained, and most of the wood was wet and useless, no way of starting a fire then.
"The wood is too wet, I can't start a fire like this" Said Merlin trying to light the fire one last time with his flint.
Arthur had already anticipated this outcome, so he just said "Well, just use you magic then"
"Alright" Said Merlin already conjuring up a fire. "I thought you'd never tell me you know"
Turns out, Arthur was't as discreet as he thought he was.