Full Idea
Reid pointed out how easily conceivable mathematical and geometric impossibilities are.
Gist of Idea
Impossibilites are easily conceived in mathematics and geometry
Source
report of Thomas Reid (Essays on Intellectual Powers 4: Conception [1785], IV.III) by George Molnar - Powers 11.3
Book Reference
Molnar,George: 'Powers: a study in metaphysics', ed/tr. Mumford,Stephen [OUP 2003], p.182
A Reaction
The defence would be that you have to really really conceive them, and the only way the impossible can be conceived is by blurring it at the crucial point, or by claiming to conceive more than you actually can