The prop. (∃x)φx . x = a : ≡ : φa can be seen to be a tautology, if one expresses the conditions of the truth of (∃x).φx . x = a, successively, e.g. by saying: This is true if so & so; that & this again is true, if so & so. ˇetc. for (∃x).φx . x = a; and then also for φy. To express the matter in this way is itself a cumbrous notation,
of wh.
wh. is what is expressed more neatly b
the a–b notation is a neater translation.