Composition, Decomposition, Relationships, Representation, Context (page 2)
The first and most fundamental concepts of mathematics are the idea of counting and the more developed idea of using a system to represent numbers. Dehaene (1997) presents the intriguing notion that several species of birds and mammals can distinguish between a single item and quantities of items. Further, there is some evidence that these birds and mammals might be able to distinguish between two objects and three objects or even between three objects and four objects. Dehaene's hypothesis, which is thoroughly developed and documented, is that certain structures within the brain seem to be specifically devoted to these primitive mathematical abilities. If this is true, then what exactly do these brain structures do? What is involved at the most fundamental level in this thing we refer to as counting?
To begin our exploration of this idea, consider two (imaginary) individuals, one who has the ability to count and one who does not. As these two individuals look out on the scenery of their lives, what do they see, and how does what they see differ? On one level we can say that what they see is exactly identical. They are both receiving the same sensory signals. What they do with those signals that they receive from their senses can differ, however. The non-counting individual probably perceives that there are multiple leaves on a tree, for instance, but beyond this perception he is unable to proceed. The counting individual is able to identify a single leaf as a unit that is repeated, he is able to momentarily focus attention on each leaf once and only once, he is able to apply a counting word to each leaf in a sequence, he is able to use the exact right sequential word for each successive leaf, and after he has focused momentarily on each and every leaf he is able to report a final "count" to tell us how many leaves there are. Furthermore, he understands that he could repeat the process using a different order of leaves but the same sequence of word-labels, and if he has been careful he will come to the same ending word each time he does this. All of this can take place without any physical evidence that he is doing anything at all; that is, it can all take place in his mind. For both individuals the scene is a single scene, but the counting individual can perform this complex analysis on the scene to provide us with some abstract information about what the scene contains. It is unlikely that he would ever do this, however, unless there were some pressing need for that information. Mathematics, while abstract, rarely makes its presence known except in response to a particular goal a person wants to achieve.
One of the most interesting parts of this counting process is the very beginning. How does one begin to count? In order for our imaginary counting person to even begin to use his ability he must first decide on an object to count. He must take what he sees as a single scene and see within it some repeated objects that he considers to be units to be counted. That is, he must mentally compose what he will consider to be units. In the example given, the countable objects were, of course, leaves. But did he have any other choices? What else could he have counted? Trees? Blades of grass? Clouds? Rocks? Any of these are possibilities. It is all a matter of choices made by the counting individual. How about clusters of leaves? What if the counter determines that each branch has, invariably, six leaves? Then, when he counts branches he can determine the total number of leaves without having to count each one. The idea of counting things in groups and then counting the number of groups is an extremely powerful idea in the development of a counting system (or a number system). In the case of counting clusters, the counter has simply composed a different unit. He has considered a branch rather than a single leaf to be his countable unit. The concept of composing various counting units is a theme that we will return to again and again.
Composition occurs in geometry as well. A key element of spatial reasoning is the ability to see a shape and mentally manipulate it. But what constitutes a shape? Is it the overall outline of the shape? Is that overall outline composed of parts? Do the parts have characteristics that can be analyzed and/or found in common? Do the parts have names? Does the overall shape have a name? Most important, can the parts and the whole be seen in more than one way? The ability to compose and manipulate units of shape is one example of the importance of composition in a geometric context.
Once we have composed a countable unit we can go in an entirely different direction. Aside from simply counting the number of objects of that unit, we could also decide to partition, or decompose, one of those units. There is an entire world of mathematics found in decomposing units. A unit can be a geometric shape, and decomposing it into parts can begin a traditional study of fractions. A unit can be a group of objects, and a consideration of a portion of the group involves a different use of fractions. If our unit is readily divided into ten equal pieces, then we can introduce the idea of decimal numbers. If we decompose a unit into two different parts and then we study the relative sizes of those parts (Are the parts equal in size? Is one of the parts twice as large as the other?), then we introduce a study of ratio. If we can readily decompose our unit into one hundred equal parts, then we can work with percents. All kinds of measurement activities and understandings rely on decomposing as well. Rarely can we accomplish the precision that we need using whole numbers of measurement units exclusively. Precision in measurement, then, relies on the idea of decomposing units into meaningful parts.
A slightly different consideration of parts and wholes brings us to the heart of addition and subtraction. When we add two quantities together we are taking two parts and composing them into a single whole. When we take a single whole (group) and break it apart into two or more smaller groups we are doing a certain kind of subtraction. When we consider addition and subtraction in this light we can begin to see them as two sides of a coin rather than as two distinctly different operations and concepts. Addition and subtraction are not the only basic operations that have their roots in decomposing. Division as well can be best understood in this light. What is division if it is not taking a whole and dividing it into smaller parts? If the whole is a dozen objects, then we can use decomposing to answer questions about smaller groups into which our dozen objects can be broken.
Decomposing, then, opens up a wide range of mathematical topics for study. Decomposing is one of our powerful ideas. On one hand we can use the idea of composing as the basis for counting, and on the other hand we can use the idea of decomposing as the basis for analysis. If composing is the key idea behind counting, decomposing is the key idea behind fractions and all of the things we can do with fractions.
Another powerful idea emerges when we begin to consider the concept of relationships in mathematics. We can study relationships between numbers or between sets of numbers. We can also study relationships between shapes and between parts of shapes. We mentioned ratios in our introduction to decomposing; the examination and analysis of ratios leads us to the powerful idea of mathematical relationships. While it is true that the concept of a relationship between numbers is completely abstract, it is, nonetheless, one of the more important topics in mathematics. An ability to see relationships as something tangible, in spite of their abstract nature, will make it possible for a student of mathematics to progress into higher levels of mathematical study. As with composing and decomposing, the concept of relationship is found in numeric as well as geometric areas of mathematics. This is part of why we consider it to be a powerful idea.
Consider the set of numbers that would be used to represent pounds of bananas and the cost of the bananas. Tthe table below presents a possible set of such numbers. An examination of the relationship between pounds of bananas and cost of the bananas reveals a consistent pattern. Multiplying the number of pounds by $0.15 yields the total cost of the bananas. Conversely, dividing the cost by $0.15 gives us the number of pounds of bananas. We say that the bananas are $0.15 per pound. The term "per" indicates a relationship. Knowing this relationship we can compute the cost for any quantity of bananas, and we can compute the weight of any given purchase. Knowing a relationship between quantities gives us a lot of power. Studying relationships is a crucial part of a study of meaningful and useful mathematics.
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