It can be of no practical use to know that π is irrational, but if we can know, it surely would be intolerable not to know.
Quoted in N Rose Mathematical Maxims and Minims (Raleigh N C 1988).
I met a man once who told me that far from believing in the square root of minus one, he didn't believe in minus one. This is at any rate a consistent attitude.
There are certainly people who regard √2 as something perfectly obvious but jib at √-1. This is because they think they can visualise the former as something in physical space but not the latter. Actually √-1 is a much simpler concept.
Algebra goes to the heart of the matter as it ignores the casual nature of particular cases.
Quoted in D MacHale, Comic Sections (Dublin 1993)