Martin Buber's work What is Man?is a work aimed at unpacking the question asked in the title. The work deals with understanding the question as opposed to answering it. Buber addresses with the question using philosophical anthropology in order to look at man in his most elemental form.
Defining man is an immeasurably large task. In undertaking such an endeavor one must look at people collectively and account for difference in every sense(sex, role, age, ethnicity, character type, etc.), but one also must look at man himself. Man must examine how he is different from other "bearers of consciousness"(64A) like animals. Man must know himself in order to define himself, which is precisely why Buber suggests the use of Philosophical Anthropology.
Philosophical Anthropology is not aimed at providing a different view-point on human existence, "it is solely intent on knowing man himself"(64B). It is a unique area of study because a man is not just the observer, but he is the subject himself, and cannot separate himself and look only at the human race, but he must contemplate himself with respect to all others. Moreover, the philosophical anthropologist cannot consider himself an object of study, but only is truly studying himself from a philosophical anthropological view if he includes his own subjectivity. He must enter completely into the act of self-reflection, and then build all knowledge of man around that core of understanding he gains from himself.
Bernhard Graethuysen defines the significance by saying that with Aristotle "man ceases to be problematic, with him man speaks of himself always as it were in the third person The special dimension, in which man knows himself as he can know himself alone, remains unentered, and for that reason man's special place in the cosmos remains undiscovered."(65B).
Mary Midgley's On Not Being Afraid of Natural Sex Differences discusses "How far are we essentially the same?"(118B).