Martian Truisms (via Valentine Michael Smith)

When he first arrives on Earth, Smith is a blank slate. He learns as he is told to by those whom he trusts. This is an example of what hapens to him.

Possibly the most controversial line in the novel. Smith is trying to understand the concept of "God", and comparing it with the Martian concept of "grokking", a word which literally means "drinking" but encompasses much more. Again, from the novel:

'Short human words were never like a short Martian word — such as "grok" which forever meant exactly the same thing. Short human words were like trying to lift water with a knife.
'And this had been a very short word.
'Smith still felt that he had grokked rightly the human word "God" — the confusion had come from his own failure in selecting other human words. The concept was truly so simple, so basic, so necessary that any nestling could have explained it perfectly — in Martian. The problem, then, was to find human words that would let him speak rightly, make sure that he patterned them rightly to match in fullness how it would be said in his own people's language.
'He puzzled briefly over the curious fact that there should be any difficulty in saying it, even in English, since it was a thing everyone knew else they could not grok alive.'

One of the most difficult concepts for Smith to grasp is the idea of humor. He has a moment of clarity when he watches monkeys in the zoo mistreat each other

This is actually a quote from Smith's friend Jubal, a great admirer of the sculptor August Rodin. Here he's discussing the artist's ability to show the truth of a human figure.

Again, a quote from Jubal, Smith's mentor and friend. Just as with the religious quote above, Heinlein's attitude toward sexuality was (and still is) considered quite radical. It's been oversimplified as the kind of "free love" espoused by caricatures of hippies in the movies, but in fact it is far more complex.

This particular quote is more general, and I think it should be more widely discussed. Just because you dislike something doesn't mean the rest of the world should be banned from enjoying it.