Full Idea
How does the author avoid reasoning in a circle when he says that we are sure that what we clearly and distinctly perceive is true only because God exists? But we can be sure that God exists only because we clearly and distinctly perceive this.
Clarification
The famous 'Arnauld Circle', nowadays known as the 'Cartesian Circle'
Gist of Idea
It is circular to make truth depend on believing God's existence is true
Source
comment on René Descartes (Meditations [1641], §5.71) by Antoine Arnauld - Objections to 'Meditations' (Fourth) 214
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.106