Can Every Ordinal Number Be Proven to Be a Transitive Set in Set Theory?

[Kontilera]

I continue with a questions regarding proofs in set theory.. :)

Halmos just writes that every ordinal number is a transitive set but doesn't prove it. Is there any simple proof of this?

 

[micromass]

Well, how did you define ordinals??

 

[Kontilera]

Haha, that was enough. :) Realized my mistake.