Every particular science carves out a domain and studies the beings within it. Physics studies natural bodies; mathematics studies quantities; medicine studies health and disease. But all of these presuppose that their objects exist — none of them investigates existence itself. Aristotle's first philosophy steps back from every particular domain to ask the prior question: what is it to be? This is what he means by "being qua being" — being studied not from the angle of any particular property but from the angle of existence as such.
"Being is said in many ways," Aristotle observes — one of the most repeated phrases in his philosophy. When we say that Socrates is, that the colour white is, that a musical event is, and that it is possible for rain to fall, we are using forms of "being" that mean genuinely different things. Aristotle identifies four main uses: being as the categories (substance, quality, quantity, relation, etc.), being as truth, being as potentiality and actuality, and being as accidental predication. First philosophy must navigate these senses without collapsing them into one.
Among all the ways of being, substance has primacy. Qualities, quantities, and relations exist, but they exist as properties of substances — the particular things that are fully real in their own right. A redness exists, but only as the redness of something. A Socrates exists in a way that his pallor does not: he is a this, an individual thing standing in the world. First philosophy is ultimately the investigation of substance — what substances are, what makes them what they are, and why they exist at all rather than nothing.
The science of being qua being is introduced in Metaphysics IV.1. Aristotle returns to the question of what substance ultimately is throughout Books VII–IX, widely regarded as the philosophical core of the work.
