"--it's probably best not to eat horses, marine mammals, or trees? But I don't think there's a list... Yet?"

Quest available: Okay So This One Is The Manual's Fault

Learn to look things up in the manual. 

Success: 10xp, ability to look things up in the manual.
Failure: More conversations like this with Wei Wuxian. 
Accept Y/N

"--or actually there might be a list?" And he accepts the quest.

Is there an obvious way to poke the manual to look things up?

To look things up in the manual, you can say "Table of Contents" for the table of contents, "search" to look for a particular term, or "index" to flip through the index. 

"Search: 'sentience'?"

Sentience, philosophical views of
Speech among nonsentient beings
Panpsychism
Sentience, evolution of
Sentience, moral views on
Sentience, history of moral views on
Sentient species on Earth
Magic use and sentience
Speech grammar for referring to sentients
Etiquette for creatures of unknown sentience
The Harkness Test

'Sentient species on Earth' seems like the relevant result here.

This opens up to a lengthy chapter which explains, in summary, that primates, parrots, horses, marine mammals, and trees are people; mammals, birds, and members of some other species such as octopuses are sentient and can be usefully talked to with the Speech but are not as sophisticated as the ones that are people; and literally everything else can be talked to up to and including thermostats and the air, but in most cases they don't have anything very useful to say.  

"Do you just want to not eat things that are definitely people, or anything that can hold a meaningful conversation?"

"If it can hold a meaningful conversation I don't want to eat it."

"That would be mammals, birds, and octopi. --Fruit is probably fine, though trees are people?"

"--wait. Trees are people? WE CUT DOWN THEIR BODIES AND TURN THEM INTO CELEBRITY GOSSIP MAGAZINES."

Quest complete: Okay So This One Is The Manual's Fault

10XP

"--Okay, yeah, that isn't good."

Now he feels bad about all the university textbooks he bought and read four pages of.

"We need to go talk to a tree right now! --We need to put on pants and then go talk to a tree right now!"

...this is a moral emergency that needs pants and dealing with now. Even if he would rather not.

And pants and shirt are put back on so there's no public indecency charge.

Hurriedly putting pants back on! Skipping the shirt and shoes because there's no time! Going to go FIND A TREE

And he can follow after him to... talk to a tree. With his vocabulary of mostly thank you and emotion words.

Here is a very nice tree of... some species. Wen Ning is not a tree expert.

Well, the manual didn't specify a specific tree species, so it should be fine?

"I can try just talking with it, though my vocabulary is pretty limited--" he flicks through the manual. "Though I do have a meditation that might help? ...if you don't mind waiting twenty minutes."

"Brought my phone."

To change something, you must first describe it. To describe something, you must first see it. Hold still in one place for as long as it takes to see something.

He can try that, even if that's a very vague instruction. He stands a reasonable distance from the tree. (Do trees have personal space?)

And he looks.

Wei Wuxian is fiddling with his phone, and Wen Ning is trying not to be too distracted by it.

The tree is more or less alone, surrounded by grass of an equally mysterious species. The side walk is covered with cracks from the heat and rain and being more trouble to repair than its worth.

It's quiet, but not silent. There's constant traffic noise, and wind rustling through leaves and making clotheslines creak. And intellectually, he knew things were never silent-- but it's something to realise how much sound he was ignoring.

And how much visual detail, too. Each clump of grass is unique, the same basic shape and parts arranged in infinite combinations, spreading out and out until it's an undefined whole but really it's all individual, unique pieces--

<<Now you're listening!>> the tree says disagreeably.

<<Sorry about the wait?>>