Full Idea
The primary faculty seems to me to be the perception of structure, that is, based upon the mirror.
Gist of Idea
Our primary faculty is perception of structure, as when looking in a mirror
Source
Friedrich Nietzsche (Unpublished Notebooks 1872-74 [1873], 19 [153])
Book Reference
Nietzsche,Friedrich: 'Unpublished of 'Unfashionable Obs' period (v 11)', ed/tr. Gray,Richard T. [Stanford 1995], p.49
A Reaction
The point about the mirror makes this such an intriguingly original idea. Personally I like very much the idea that structure is our prime perception. See Sider 2011 on structure.