The main limitation of formal logic is that it applies to matters which are either true or false, and which can be known, at least in theory, with some degree of certainty. These are not, however, the only matters we discuss, or the ones concerning which discussion is most useful. If, for example, Lawyer A is defending a client who can be proven to have been several thousand miles away from  a stabbing victim, Lawyer A has an easy job. If we are arguing about the best healthcare system or the relative merits of two poems or the morality of euthanasia, we are often dealing with very fuzzy concepts (what constitutes life, what is the nature of the good life, to what degree art should delight or instruct) and these are not really issues of true or false, of the sort described by formal logic, but closer to the types of issues on which we use informal logic or rhetoric to persuade rather than to prove.