ABSTRACT
VI.1 The formal objects under discussion in mathematical logic include such things as letters, forms and various kinds of tableaux. Forms are expressions, i.e. finite sequences of letters (A.5); and tableaux are finite sequences of finite sequences of forms (A.7). For instance, if p and q are sentence-letters, the sentence-form is the finite sequence
of length 14, each of whose terms is a letter. (A form that consists of a letter alone is the finite sequence of length 1 whose only term is that letter.) Again, the tableau
is the finite sequence of length 2 whose first term
is the left-hand branch, and whose second term
is the right-hand branch; each branch being itself a finite sequence of sentence-forms.
