I think the reader is beginning to get an inkling of the crucial importance of the “number of” function in Peirce’s way of looking at logic. It is one plank in the bridge from logic to the theories of probability, statistics, and information, in which setting logic forms but a limiting case at one scenic turnout on the expanding vista. It is one of the ways Peirce forges a link between the eternal, logical, or rational realm and the secular, empirical, or real domain.
With that note of encouragement and exhortation, let us return to the details of the text.
But not only do the significations of and here adopted fulfill all absolute requirements, but they have the supererogatory virtue of being very nearly the same as the common significations. Equality is, in fact, nothing but the identity of two numbers; numbers that are equal are those which are predicable of the same collections, just as terms that are identical are those which are predicable of the same classes. So, to write is to say that 5 is part of 7, just as to write is to say that Frenchmen are part of men. Indeed, if then the number of Frenchmen is less than the number of men, and if then the number of Vice‑Presidents is equal to the number of Presidents of the Senate; so that the numbers may always be substituted for the terms themselves, in case no signs of operation occur in the equations or inequalities.
Peirce observes that the measure on logical terms preserves the relations of implication or inclusion which impose an ordering on those terms. Here Peirce uses a single symbol to denote the linear ordering on numbers, but also what amounts to the implication ordering on logical terms and the inclusion ordering on classes. Later he will introduce distinctive symbols for the logical orderings. The links among terms, sets, and numbers can be pursued in all directions and Peirce has already indicated in an earlier paper how he would construct the integers from sets, that is, from the aggregate denotations of terms. I will try to get back to that another time.
We have a statement of the following form.
If then the number of Frenchmen is less than the number of men.
This goes into symbolic form as follows.
In this setting the on the left is a logical ordering on syntactic terms while the on the right is an arithmetic ordering on real numbers.
The question that arises in this case is whether a map between two ordered sets is order-preserving. In order to formulate the question in more general terms, we may begin with the following set-up.
Let be a set with the ordering
Let be a set with the ordering
An order relation is typically defined by a set of axioms that determines its properties. Since we have frequent occasion to view the same set in the light of several different order relations, we often resort to explicit specifications like and so on to indicate a set with a given ordering.
A map is order-preserving if and only if a statement of a particular form holds for all and in namely, the following.
The “number of” map has just this character, as exemplified in the case at hand.
The on the left is read as proper inclusion, in other words, subset of but not equal to, while the on the right is read as the usual less than relation.
- Peirce’s 1870 Logic of Relatives • Part 1 • Part 2 • Part 3 • References
- Logic Syllabus • Relational Concepts • Relation Theory • Relative Term