Full Idea
I can object to your walking on the grass by asking 'What if everybody did that?', but the advantages of cheating depend upon the fact that most people don't cheat, so justifying my own cheating must involve special pleading.
Gist of Idea
'What if everybody did that?' rather misses the point as an objection to cheating
Source
Gordon Graham (Eight Theories of Ethics [2004], Ch.6)
Book Reference
Graham,Gordon: 'Eight Theories of Ethics' [Routledge 2004], p.114
A Reaction
It is, of course, reasonable to ask 'What if everybody cheated?', but it is also reasonable to reply that 'the whole point of cheating is that it exploits the honesty of others'. This shows that Kant cannot simply demolish the 'free rider'.