Full Idea
Cyclists are not essentially two-legged (a one-legged cyclist exists, but can't cycle any more), and mathematicians are not essentially rational (as they can lose rationality and continue to exist, though unable to do mathematics).
Gist of Idea
Cyclist are not actually essentially two-legged
Source
comment on Willard Quine (Word and Object [1960], §41.5) by Baruch Brody - Identity and Essence 5.1
Book Reference
Brody,Baruch: 'Identity and Essence' [Princeton 1980], p.86
A Reaction
Was Quine thinking of the nominal essence of this person - that 'cyclists' necessarily cylce, and 'mathematicians' necessarily do some maths? It is as bad to confuse 'necessary' with 'essential' as to confuse 'use' with 'mention'.