Marcus then responds positively to the question:
Must a man then be one of these, who act like this without observing it?
The next part of the text is not completely clear and seems to differ subtlely between the Long, Hard and Staniforth translations. My interpretation is that Marcus deals with an objection from his interlocutor, that as a social being one necessarily knows one is behaving prosocially:
But this very thing is necessary, the observation of what a man is doing: for, it may be said, it is characteristic of the social animal to perceive that he is working in a social manner, and indeed to wish that his social partner also should perceive it.
Marcus acknowedges this as true, yet claims it still falls short of the full truth.
It is true what you say, but you do not rightly understand what is now said: and for this reason you will become one of those I spoke of before, for even they are misled by a certain show of reason. But if you choose to understand the meaning of what is said, do not fear that for this reason you will omit any social act.
In other words (I think) we have a delicate balancing act to perform. Prosocial behaviour is a conscious philosophical choice, yet we need to get into the habit of doing it unconsciously – or perhaps not let it “go to our heads”?