I was reading Jech's Set theory; there after introducing Russell's Paradox, he asserted:
The safe way to eliminate paradoxes of this type is to abandon the Schema of Comprehension and keep its weak version, the Schema of Separation...
Once we give up the full Comprehension Schema, Russell’s Paradox is no longer a threat...
Replacing full Comprehension by Separation presents us with a new problem. The Separation Axioms are too weak to develop set theory with its usual operations and constructions. Notably, these axioms are not sufficient to prove that, e.g., the union $X \cup Y$ of two sets exists, or to define the notion of a real number.
Why did he say the Separation axioms are too weak 'to develop set theory with its usual operations and constructions'?
Maybe it might be trivial, but I'm not able to comprehend the point. Can anyone shed some light on the author's statement?