Grin's

“What are then Samurai?”

For a long while, this sentence Takasugi said to Katsura during their dialogue in Benizakura arc, I thought it was funny and I used to think “dude you should know”.

But after some time, I thought it could be way more meaningful than a purely rhetorical/philosophical question.

And I've got the answer when I noticed something peculiar.

So, I am sharing this idea I had for a while, in case somebody had the same initial reaction as me. (Of course, it is my way to view and interpret it, which could be right or wrong.)

"Katana kill, master smiths forge them, Samurai... What then are samurai? 

An object that exists for a single purpose is strong, flexible and beautiful." 

Apparently, he is being philosophical and rhetorical, nothing more. But he is talking to Zura at the moment they are facing the fact that their paths are now drifting apart, because of Zura's “betrayal” (from Takasugi's point of view) with his political change of heart in his new moderate stance and the acceptance of a world he can't put up with, like Zura sometime before. 

Then he basically tells Zura they can't but destroy that world for having taken Shouyou, who set the foundation for their world teaching them his bushido, away from them, and adds his objective has never changed. 

Let's now consider the moment we get acquainted with Shouyou's bushido. 

“What is a Samurai? Would you tell me? […] I am not the kind of Samurai you know.” and then goes on, declaring his Bushido to an amazed little Shinsuke. 

Since before Shouyou Takasugi and Katsura were looking for their definition and path of being a Samurai, they were fed up with that academic, static, education of what a samurai “officially” was. Sensei provided them with a wide, fluid and original representation of “being a samurai”, fulfilling a need they had, their place in the world. (It is interesting so that those then-little-samurai are now confronting with the topic in their adult days.)

Takasugi also says “An object that exists for a single purpose is strong, flexible and beautiful” talking about katana, but... Maybe the bushido Shouyou had given them as well as considering the adjectives he used might as well describe it, not only the sword. 

Translation and actual wording may vary from one another, but both Takasugi at Zura and Shouyou at Takasugi are asking the same question: what is it a samurai (or what he does, or what's his purpose, if you prefer).

So, Shinsuke with that rhetorical question on the ship was not being merely philosophical but appealing to Zura's memories and emotions stirring them up, to make him understand his view.

And, above all, he is pointing out he's not straying from Sensei's teaching (Zura openly confirms it in Rakuyou).