Full Idea
The contents of definition by division are the so-called primary genus (such as 'animal') and the differentiae. ...It should always be prosecuted until the level of non-differentiation is reached, ...and the last differentia will be the substance.
Clarification
'Differentiae' are what makes it different
Gist of Idea
Definition by division is into genus and differentiae
Source
Aristotle (Metaphysics [c.324 BCE], 1037b30)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.212
A Reaction
[Second half at 1038a14] Note that this is only one type of Aristotelian definition, the 'definition by division'. The aim of this type of definition is to analyse down to substance. Presumably you can't ignore crucial features found on the way?