Quote 1
"I see you're looking at my feet," he said to her when the car was in motion.
"I beg your pardon?" said the woman.
"I said I see you're looking at my feet."
"I beg your pardon. I happened to be looking at the floor," said the woman, and faced the doors of the car. (2.2-5)
This is the first time that Seymour seems strange to us. What's so interesting is that his behavior hasn't changed at all since he was joking around with Sybil. While that sort behavior may be perfectly acceptable around children, it appears mad, or crazy, in the adult world.
Quote 2
Ask me something else, Sybil," he said. "That's a fine bathing suit you have on. If there's one thing I like, it's a blue bathing suit."
Sybil stared at him, then looked down at her protruding stomach. "This is a yellow," she said. "This is a yellow."
"It is? Come a little closer." Sybil took a step forward. "You're absolutely right. What a fool I am." (20.20-22)
At this point, we know that Seymour is mentally unstable because of Muriel's conversation with her mother. So all his comments to Sybil leave us guessing: is he really unstable, or just playfully joking with this little girl?
Quote 3
"Yes. Yes, I do," said the young man. "What I like particularly about her is that she never does anything mean to little dogs in the lobby of the hotel. That little toy bull that belongs to that lady from Canada, for instance. You probably won't believe this, but some little girls like to poke that little dog with balloon sticks. Sharon doesn't. She's never mean or unkind. That's why I like her so much."
Sybil was silent. (2.64-5)
Passages like this one can convince us of Seymour's sanity. He's clearly joking with Sybil, as any adult who is great with children might. He even takes an opportunity to reprimand her – if playfully – as an authoritative adult.