Full Idea
By ideas I do not mean images such as are formed at the back of the eye, or in the midst of the brain, but the conceptions of thought.
Gist of Idea
Ideas are not images formed in the brain, but are the conceptions of thought
Source
Baruch de Spinoza (The Ethics [1675], II Pr 48)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.120
A Reaction
This appears to be equating 'ideas' with what we now call 'concepts', which presumably makes Spinoza less open to criticism than other philosophers of his time, for postulating baffling mental copies of the world.