selfAdjoint was talking about 'free' choice, in other words a kind of choice that is made free of deterministic constraints. The argument is that it's impossible to demonstrate the existence of 'free' choice (vis a vis 'deterministic' choice), because there is nothing in the nature of any given act that will tell you, of itself, whether or not it was chosen 'freely.' If you swallow motor oil, that may have been an act of free choice, or it may have been caused by a deterministic set of events (eg, a certain conversation caused you to want to demonstrate that you have free choice, and you deterministically chose to swallow motor oil in an effort to demonstrate this, and then you deterministically carried out the act).
So, if it's true that the existence of free choice cannot be thus demonstrated, then selfAdjoint argues that one can only flatly state that it exists (or does not exist), and this is called 'assertion.' (I think we can do better justice to the problem than just assertion, but that's a topic for another thread.)