Adler's purpose in distilling the philosophy of Aristotle for modern readers is to help us think more clearly. The beauty of Aristotle in his opinion is the way he used common human experiences and notions common to all of us - thing, body, mind, change, cause, etc. - to reason about our existence. These commonalities are accessible by all, unlike the scientific experimentation that goes on behind the doors of a laboratory and are only read about by non-specialists such as ourselves.
Categorizing things is where Adler begins his exploration of Aristotle. Categorizing is so basic to our human existence that it's practiced by children playing "Guess Who?"
The most basic categories into which we divide things (or bodies, to use Aristotle's terminology) are living and non-living. All living things (bodies) take nourishment, grow, and reproduce while inanimate objects (bodies) do not. We can subdivide living bodies into the categories of plant or animal. Granted, there seem to be bodies on the borderline between living and non-living or between plant and animal; some things (bodies) have characteristics that seem to straddle the dividing line.
That we have borderline cases is actually good though. Unless our minds recognized the clear-cut distinctions between something like a rock and a rabbit, we wouldn't have to puzzle over the borderline cases. But the human mind does recognize these larger categories, and that sets us apart from all other animals! Other animals show intelligence, but they do not ask one another questions about the nature of things and work to answer them.
Our minds recognize these categories because it is able to discern the nature possessed by all the individual things making up that category, or class. Artistotle discerned five main classes of physical things and placed them in an ascending order. Each subsequent class possesses the characteristics of that which comes before it in the chain, as well as some new characteristic that sets it apart:
Inanimate Bodies1) Elementary Bodies - have weight, occupy space, and are composed of only one element.
2) Composite Bodies - have weight, occupy space, and are composed of two or more elements
Animate Bodies3) Plants - have all the characteristics of composite bodies, but also take nourishment, grow, and reproduce
4) Animals - do this, as well as move and employ additional powers of sense such as touch, sight, hearing, taste, and smell.
5) Human beings - in addition to all that has come before, possess rational thought.
The major differences indicated above, that separate one class of bodies from another, are essential differences. All the other differences within a class are superficial, or accidental differences. People have tried to divide our class into many different sub-classes (race, height, hair color, I.Q., place of origin, etc.), but these are all accidental differences. Rational thought (asking "what" and "why") is the common property of human nature; it's what sets us apart as a class unto ourselves; all the other differences between us are superficial.