Logic and Mathematics - Revision Notes

Natural Numbers=Words used to count things

3 Basic Approaches to Number:
1) They are natural and can be empirically observed (MILL)
2) They are institutions of a perfect and harmonic platonic world (PYTHAGORAS, DESCARTES)
3) They are abstract logical objects, constructed purely from syntax (FREGE)

1) Numerical Naturalism:
Stone age tribes appear to be able to judge simple empirical plurality. “one thing”, “more than one thing”, “lots of things” are all the numbers they need. If you walk into a room and see one person you don’t physically count that one person, you can just categorise it in terms of plurality. Most people can get up to six or even seven before they physically would have to count how many people there are.  A large number like 7,246 is just a predicate symbol of more basic symbols, organised according to known syntax.  Realistically you would just say “there are a lot of people” or “the room is full”.
MILL went beyond his predecessors claiming that not only all science, but also all mathematics is derived from experience. The definition of each number contains the assertion of a physical fact. Every number (2, 3, 4 etc.) denotes physical phenomena and connotes a physical property of that phenomena. E.g. “two” denotes a pair of things, and connotes what makes them pairs. Two apples are physically distinguishable from three apples. They are a different visible and tangible phenomenon.

Mill doesn’t make it clear exactly what the property is that is connoted by the name of a number, and Mill also admits that the mind has some difficulty distinguished between 103 apples and 104 apples.

2) Pythagoreanism/Platonism:
Prime numbers are pre-existing, eternal, supernatural forms.  They are necessary preconditions for consciousness. This goes against KANT’S theory “existence is not a predicate”, for Platonism existence is a predicate of numbers.  Prime numbers exist in a non-human dimension, just like the perfect form of an object exists in the realm of the forms. These things are eternally true.
There is a special religious significance to the number three. Three is the magic number.  Rule of thirds, three part drama, three chord triad etc.
PYTHAGORAS and all the Greeks only regarded plurals as natural numbers, so began counting with two. “One” and “not one” were different logical categories. FREGE later points out this can cause a problem in logic, “there is no one on the road” does not mean the road is empty.

PROBLEM OF NOTHING AND ZERO – Introduction of zero came from India after the fall of Rome.  This is difficult because zero=nothing=something. This falls under ARISTOTLE’S law of contradiction. LEIBNIZ solves this law of contradiction by stating that an object can contain its own negation. Modern philosophers of mathematics have now asserted that zero is in fact a natural number.

3) Numbers as Logical Objects:
The problem of nothing and zero remained unsolved for 1000 years until FREGE.
He links logic and arithmetic in an overall system of philosophy of language. He attempted to demonstrate the logical basis for numbers therefore refuting Platonism. He also rejected MILL’S numerical empiricism, you cannot find zero in nature.
FREGE’S method:
Axiom= all things that are identical are equal to themselves (definitional, a priori, deductive truth).
- All things which are pairs are identical to other pairs.
- We assign a nominal symbol to this class of pairs (e.g. two)
- “One” is the class of all things not associated with other things.
- “Zero” is the class of all possible objects that are not equal to themselves.  “Null class”
- Therefore “zero” is defined into existence as a logical object.