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Full Idea
Individual instances of a kind of phenomenon, in Aristotle's view, can only be perceived through sense-perception; but they are not the proper subject-matter of scientific demonstration and definition.
Gist of Idea
Individuals are perceived, but demonstration and definition require universals
Source
Kathrin Koslicki (Essence, Necessity and Explanation [2012], 13.3.1)
Book Ref
'Contemporary Aristotelian Metaphysics', ed/tr. Tahko,Tuomas [CUP 2012], p.198
A Reaction
A footnote (11) explains that this is because they involve syllogisms, which require universals. I take Aristotle, and anyone sensible, to rest on individual essences, but inevitably turn to generic essences when language becomes involved.