Well, "slithy" means "lithe and slimy." "Lithe" is the same as "active." You see it's like a portmanteau - there are two meanings packed up into one word.
May we not then sometimes define insanity as an inability to distinguish which is the waking and which the sleeping life? We often dream without the least suspicion of unreality: 'Sleep hath its own world', and it is often as lifelike as the other.
It’s a miserable story!” said Bruno. “It begins miserably, and it ends miserablier. I think I shall cry. Sylvie, please lend me your handkerchief.” “I haven’t got it with me,” Sylvie whispered. “Then I won’t cry,” said Bruno manfully.
She [Alice] went on "And how do you know that you're mad?" "To begin with," said the Cat, "a dog's not mad. You grant that?" "I suppose so," said Alice. "Well, then," the Cat went on, "you see, a dog growls when it's angry, and wags it's tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."
If you set to work to believe everything, you will tire out the believing-muscles of your mind, and then you'll be so weak you won't be able to believe the simplest true things.