Full Idea
It is not sufficient for Descartes to claim that he perceives something in himself clearly and distinctly, for this is to not complete the demonstration, unless he shows the method through which others can attain the same experience.
Gist of Idea
Descartes needs to demonstrate how other people can attain his clear and distinct conceptions
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], App X)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.39
A Reaction
For the simplest rational insight this seems a rather tough requirement. If you say A>B, and B>C, so A>C, then once you have grasped the concept of 'greater than' I'm not sure there is a further possible demonstration.