Mark English in Scientia Salon:
Expertise implies epistemic authority: the expert — by definition — speaks with authority within his or her area of expertise. If the expertise is recognized, the authority automatically follows and doesn’t have to be claimed or argued for.
But the word “expertise” normally applies only to reasonably narrow, clearly defined and recognized areas of knowledge, theoretical or practical. And general philosophy (encompassing all the traditional sub-disciplines) is just too broad and ill-defined for the word to apply in any natural or straightforward sense. Its meaning must be, as it were, stretched to fit.
There is even disagreement about what philosophy is about — or if it is about anything at all.
Some see it as the normative study of rationality. But it is simply not plausible in my view that philosophy (or any single discipline) could effectively encompass the entire realm of reason or rationality. Logic perhaps, but reason is a much broader concept.
Massimo Pigliucci prefers to see philosophy as being concerned with the exploration of conceptual as distinct from empirical space (the sciences being focused on the latter)[1]. But, again, conceptual space is just too vast an area to be subsumed by any one discipline. Besides, unconstrained by empirical (or mathematical) considerations, conceptual space is really not all that interesting. And of course, the sciences are just as much about model-building (i.e., exploring conceptual space) as they are about empirical evidence; and mathematics is pretty much all about exploring conceptual spaces of certain kinds.
More here.