Empiricists (I am told, though I'm not convinced I've ever really met one) believe that what is particularly disclosed in sense-experience is all that is objectively true. Universal affirmations and knowledge of essence are abstractions from sense-experience that do not specifically correspond to anything outside the mind, and are therefore untrue insofar as they cannot correspond to any physically "real" object. However, it is not clear that sense-experience is ever given directly in sense-experience. Indeed, it rather seems as though sense-experience is a universal. If this is so, then the belief that universals cannot correspond to particulars disclosed through sense-experience would appear to be a contradiction in terms, since it would immediately negate the possibility of its correspondence to anything objectively real.
It is not advisable to restrict possible evidence against one's position in advance. For as Socrates says in Republic to Thrasymachus, we cannot be certain in advance whether or not what we are seeking may lie within the area which we have closed off. Now, we can close off possible realms of evidence in two ways: a provisional way, and an absolute way. We close off evidence provisionally if the closure is temporary and methodological. The closure is absolute if a whole realm of phenomena are determined (extrinsically to the matter under consideration) to be off limits. It would seem that the absolute closure of the non-empirical in empiricism is problematic precisely because it is using an a priori concept (the empirical) to argue against the reality of a priori concepts.
It is not clear exactly what material means to empiricists. Dr. Johnson kicking the rock seems to be about as deep as the reflection on this point goes. There is a reason for this. If they deny that materiality has any objective meaning, then they open themselves up to the question of how they have decided to exclude immaterial objects. But if they admit that materiality does mean something, what could it mean but the essence of the material? And if material has essence, then their whole position seems to unravel rapidly.