Proportions of Numbers and Magnitudes In the Elements, Euclid devotes a book to magnitudes (Five), and he devotes a book to numbers (Seven). Both magnitudes and numbers represent quantity, however; magnitude is continuous while number is discrete. That is, numbers a recomposed of units which can be used to divide the whole, while magnitudes cannot be distinguished as parts from a whole, therefore; numbers can be more accurately compared because there is a standard unit representing one of something. Numbers allow for measurement and degrees of ordinal position through which one can better compare quantity. In short, magnitudes tell you how much there is, and numbers tell you how many there are.
This is cause for differences in comparison among them. Euclid's definition five in Book Five of the Elements states that ' Magnitudes are said to be in the same ratio, the first to the second and the third to the fourth, when, if any whatever be taken of the first and third, and any whatever of the second and fourth, the former exceed, are alike equal to, or alike fall short of, the respectively taken in corresponding order.' From this it follows that magnitudes in the same ratio are proportional. Thus, we can use the following algebraic proportion to represent definition 5. 5: (m) a: (n) b: : (m) c: (n) d. However, it is necessary to be more specific because of the way in which the definition was worded with the phrase 'the former alike exceed, are alike equal to, or alike fall short of...
.' . Thus, if we take any four magnitudes a, b, c, d, it is defined that if m is taken of a and c, and n is taken of c and d, then a and b are in same ratio with can d, that is, a: b: : c: d, only if: (m) a > (n) b and (m) c > (n) d, or (m) a = (n) b and (m) c = (n) d, or (m) a < (n) b and (m) c < (n) d. Though, because magnitudes are continuous quantities, and an exact measurement of magnitudes is impossible, it is not possible to say by how much one exceeds the other, nor is it possible to determine if a > b by the same amount that c >d. Now, it is important to realize that taking is not a test to see if magnitudes are in the same ratio, but rather it is a condition that defines it. And because of the phrase 'any whatever,' it would be correct to say that if a and b are in same ratio with c and d, then any one of the three instances above, m and n being 'any whatever,' are true. Likewise, as stated in proposition 4, the corresponding are also in proportion.
It would be incorrect, however; to say that are taken of the original magnitudes to show that they are in same ratio. The two instances coexist. Furthermore, if there is any one possibility of taking ' whatever,' and not having any one of the above three instances come true, then the instance is not that of same ratio, but rather that of greater or lesser ratio as is stated in definition 7, Book 5. In Book Seven, number replaces magnitude as the substance of ratios and proportions. A number is a multitude composed of units. Definition 20 states that 'Numbers are proportional when the first is the same multiple, or the same part, or the same parts, of the second that the third is of the fourth.' Thus, there are three instances of numerical proportions: same multiple- 18: 6: : 6: 2 same part- 2: 4: : 4: 8 same parts- 5: 6: : 15: 18.
Compared to the definition of proportion in Book 5, this one is much less complex and more easily comprehended because using numbers is more exact and concrete. First of all, there is no taking of of the antecedents and consequent's of two ratios. This is because the taking of is a necessary condition when it is only possible to say that one magnitude is greater, lesser, or equal to another. With numbers, however; there is a more specific relationship. Two is less than five by three units. It is necessary to state by how many, which then limits the comparison.
For instance, in the example above of 'same multiples,' one can see that eighteen is three multiples of six and that six is three multiples of two. Thus, the phrase '... alike exceeding, alike equal to, or alike falls short of... .' is replaced with '... same multiple, same part, or same parts... .' Numbers are representations of magnitude.
They are more easily compared, but the proportion of numbers is fundamentally the same as that of magnitudes, since a proportion is generally a similarity between ratios. A proportion of numbers is therefore included in the proportion of magnitudes as a specific case.