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Topic: [HM] Dedekind's objection to the Newtonian concept of number.
Replies: 23   Last Post: Aug 2, 2006 12:39 AM

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 Robert (Bob) Eldon Taylor Posts: 101 Registered: 12/3/04
[HM] Dedekind's objection to the Newtonian concept of number.
Posted: May 26, 2005 1:15 PM

Dear Julio and All,

In his Stetigkeit Dedekind raises an objection to the then prevalent
Newtonian concept of number, that it depends on the confused and ill
defined concept of magnitude. In a footnote he raises the further
objection that, after all, it could not be extended to complex numbers,
presumably because there are no "complex magnitudes". If there are no
complex magnitudes then we cannot speak of the ratio of a complex
magnitude to a unit magnitude.

This statement has always bothered me and I have tried to get around
it. But first let us return to the main objection. There has been no
attempt that I am aware of to provide a general definition of magnitude
and certainly such a definition would be very difficult as can be seen
from the following incomplete and very rough definition.

What is meant by "magnitude" is the incomplete abstraction of a quality
of a geometric figure which is independent of the location of the
figure. By incomplete is meant that when we speak of adding,
subtracting, multiplying magnitudes we must always refer to the
geometric object and construct the sum, difference or product. In the
case of ratio, however, we do not refer to the geometric object. A
ratio is not constructed because not a geometric object but a being of a
different sort entirely.

Now given a line AB as unit and another line CD, according to Newton's
definition, the ratio of CD to AB is a number. If a whole number, then
AB multiplied, that is made multiple, that many times is the length CD.
If a fraction, then we must first subdivide AB to create a unit which
can measure CD. If neither a whole number nor a fraction, then we must
appeal to more powerful techniques, which, we being Newton, we are
confident we have available.

Theoretically, the same number can be determined using instead areas,
volumes or any other sort of magnitude for which ratios are defined,
but, since Descartes at least, usually lines have been used because the
geometric constructions are simpler. These magnitudes then are lines,
fixed in size, but abstracted from their location and movable at will.
Since in the traditional concept a line has two distinct endpoints, the
notion of a zero magnitude is impossible, so we have defined positive
real numbers (non zero). In this view, multiplication has been extended
to include the stretching or shrinking of the unit AB to match the size CD.

To introduce zero and negative numbers we introduce the concept of
directed line segment or directed magnitude. Take a directed line OA as
unit and, on the same line, any other directed line OB. The ratio OB
to OA then defines a real number. If B is on the same side of O as A,
the number will be positive, if on the opposite side, negative. We
agree that A must be distinct from O but B can coincide with O in which
case the number is zero. We have now to extend the concept of
multiplying the unit OA by zero and by a negative number

To define complex numbers, we need only release the restriction that OA
and OB be in the same line. Multiplication (of a directed line) by a
complex number involves a stretching (or shrinking) s and a rotation r.
If a unit line OB is erected perpendicular to the unit line OA then the
stretching s = 1 and the rotation is pi/2 and we may call the ratio i
and see immediately that i squared is -1.

So from this view a complex number is the ratio of a directed line
segment to a directed line segment not in the same line which is taken
as a unit. Since, with the substitution of directed line segment for
magnitude, this definition is essentially the same as Newton's, it seems
to me Dedekind's secondary objection fails, although his primary
objection, i.e. that the concept of magnitude is very murky and ill
defined, still stands. (However my objection that the abandonment of
the Newtonian concept may have been premature and that the modern
concept is quite as murky as that of magnitude, also still stands. I do
not see that the concept of directed magnitude or directed line segment
is any more murky than that of magnitude.)

I have tried to extend the above analysis to quaternions with very
limited success.

Regards from Comer, Georgia where the weather has been painfully beautiful,
Bob

Robert Eldon Taylor
philologos at mindspring dot com