Full Idea
Concepts are of such a nature that we can make some of them ourselves a priori, without standing in any immediate relation to the object; namely concepts that contain the thought of an object in general, such as quantity or cause.
Gist of Idea
Some concepts can be made a priori, which are general thoughts of objects, like quantity or cause
Source
Immanuel Kant (Prolegomena to Any Future Metaphysic [1781], 282)
Book Reference
Kant,Immanuel: 'Prolegomena to Any Future Metaphysic', ed/tr. Lucas,Peter G. [Manchester UP 1971], p.37
A Reaction
'Quantity' seems to be the scholastic idea, of something having a magnitude (a big pebble, not six pebbles).