Full Idea
I would go further than you and note that all our ideas seem to be adventitious - to proceed from things which exist outside the mind and come under one of our senses. ..The idea of a giant is a man of ordinary size which the mind enlarges at will.
Gist of Idea
All ideas are adventitious, and come from the senses
Source
comment on René Descartes (Meditations [1641], §3.38) by Pierre Gassendi - Objections to 'Meditations' (Fifth) 280
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.78
A Reaction
A classic early statement of modern empiricism. Gassendi needed to think about logic, maths, and necessities to make his case more secure. Where did his idea to 'enlarge' the giant come from?