 We have indicated how the condition of necessity can be interpreted in the proof-centered approach. The condition of formality can be accounted for as well. Note that on the present perspective as well, there is a division of the vocabulary into logical and nonlogical. This division can be used to define substitutions of an argument. A substitution of an argument is an argument obtained from the original one by replacing the nonlogical terms with terms of the same syntactic category in a uniform manner. A definition of validity that respects the condition of formality will entail that an argument is valid if and only if all its substitutions are valid, and in the present context, this is a requirement that there is a proof of all its substitutions. This condition is satisfied in any proof system where rules are given only for the logical vocabulary. Of course, in the proof-centered approach as well, there is a question of distinguishing the logical vocabulary (see the entry on  logical constants).