Quote by sigurdW
There is no set containing all sets, but there is a class containing all sets...Mathematicians say.

That I am talking about , the book haven't considered classes yet ( I know some about them ) , but the question is when I say for all x such that P(x) , will it mean "if you have proved the existence of set x then P(x) holds" or what , and whwn I say there exists some x such that P(x) holds , will it mean " It can proven the existence of a set x in ZFC such that P(x)" , Here I consider the statements " It can be proven" and alike are informal . So Can any one explain to me What it is meant by them logically.