There seem to be two kinds of predicates -- those which ask about what
an object is like, and those which ask about what it is; e.g:

obj.respond_to? :m # does obj respond to :m?
obj.include? item  # does obj include item?
obj.all? { cond }  # do all members of obj meet cond?

obj.is_a? Class        # is obj a Class?
obj.instance_of? Class # is obj an instance of Class?
obj.nil?               # is obj nil?

#start_with? seems to fit into the first category, and to be consistant
with the grammar of that category:

obj.start_with? 'string' # does obj start with 'string'?

Regards,
Jordan