What does Marvel tell himself in regard to the incident? How is he convinced otherwise?

Marvel tells himself that he must be drunk. He is convinced otherwise when the invisible man begins to throw flints at him.

"Very well," said the Voice, in a tone of relief. "Then I'm going to throw flints at you till you think differently."

"But where are yer?"

The Voice made no answer. Whizz came a flint, apparently out of the air, and missed Mr. Marvel's shoulder by a hair's-breadth. Mr. Marvel, turning, saw a flint jerk up into the air, trace a complicated path, hang for a moment, and then fling at his feet with almost invisible rapidity. He was too amazed to dodge. Whizz it came, and ricochetted from a bare toe into the ditch. Mr. Thomas Marvel jumped a foot and howled aloud. Then he started to run, tripped over an unseen obstacle, and came head over heels into a sitting position.