Rigour

"A thinking that approaches it objects openly, rigorously ... is also free toward its objects in the sense that it refuses to have rules prescribed to it by organized knowledge. It ... rends the veil with which society conceals them, and perceives them anew."

"Mathematical knowledge is no more rigorous than philological-historical knowledge. It merely has the character of “exactness,” which does not coincide with rigor."

"Anthony Zee, Quantum Field Theory in a Nutshell, 2nd edition (2010), p. 23; 1st edition (2003)"

"Physicists are sloppy about mathematical rigor, but even so, they have to careful once in a while to make sure what they are doing actually makes sense."