When you talk of a thing being deemed to be something, you do not mean to say that it is that which it is to be deemed to be. It is rather an admission that it is not what it is to be deemed to be, and that, notwithstanding it is not that particular thing, nevertheless it is to be deemed to be that thing.
Classic legalese, Justice Cave in R v Norfolk, 60 LJQB 379 (1891)