Description: If a class is a member of another class, then it is a set. Theorem 6.12 of Quine p. 44. (Contributed by NM, 26-May-1993) (Proof shortened by Andrew Salmon, 8-Jun-2011) (Proof shortened by Wolf Lammen, 28-May-2025)
Ref | Expression | ||
---|---|---|---|
Assertion | elex | |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | elissetv | |
|
2 | isset | |
|
3 | 1 2 | sylibr | |