Full Idea
It has long since been noticed that in all substances the subject proper, namely what is left over after all the accidents (as predicates) have been taken away and hence the 'substantial' itself, is unknown to us.
Gist of Idea
The substance, once the predicates are removed, remains unknown to us
Source
Immanuel Kant (Prolegomena to Any Future Metaphysic [1781], 333)
Book Reference
Kant,Immanuel: 'Prolegomena to Any Future Metaphysic', ed/tr. Lucas,Peter G. [Manchester UP 1971], p.97
A Reaction
This is the terminus of the process of abstraction (though Wiggins says such removal of predicates is a myth). Kant is facing the problem of the bare substratum, or haecceity.