 On the views against which Brewer is positioning his own, actions are motivated by desires, which are propositional attitudes: that is, centrally, descriptions of states of affairs that might be realized in the future, and in light of which you effect and adjust what you can and are willing to, so as to make the description true.  (Brewer attempts to reappropriate the concept of desire for his own purposes, but I won’t spell that move out here.)  However, what you are moved – as opposed to ‘motivated’ – by, in the course of a dialectical activity, cannot be captured by a proposition.  For one thing, a proposition has a definite, stable content; propositions are sometimes thought of as partitioning all the possible worlds – the ways things might be or might have been – into those that are, intuitively, compatible with it, and those that are not; the proposition effects its partition once and for all.  But the content of what moves you, in the course of dialectical activity, is deepening and evolving, and there is no point at which apprehending it is properly over.  So no proposition could successfully demarcate what it is the activity is after and about.