Q: Confusion on union and intersection of sets I am studying set theory on my own, using the book of Enderton: Elements of Set Theory. My question is the following:
On Chapter $2$, the book discusses Arbitrary Unions and Intersections. It starts by treating a "general case", which is a set $A$ in which every $a_j$ in $A$ is also a set.
I understand the union operation, for example when we are looking at the union of $\cup \lbrace \lbrace 2,4,6 \rbrace, \lbrace 6,16,26\rbrace,\lbrace 0\rbrace \rbrace = \lbrace2,4,6,16,26,0\rbrace$. 
Now suppose we want to compute $\lbrace a,b,c,d\rbrace$. The result in the book is $a\cup b \cup c\cup d$. What it confuses me is that the axiom says that the union is a set (s.t. etc.), the above operation is meaningless unless $a,b,c,d$ are sets themselves. Is this correct?
A similar question is the calculation in the book $\cap\lbrace a,b\rbrace =a\cap b$.
