P l a n e G e o m e t r y An Adventure in Language and Logic based on ## I N T R O D U C T I O N## Geometry: The Study of FiguresGEOMETRY, which literally means land measurement, is the study
of figures. A circle is a figure, a triangle is a figure. A figure is whatever has a boundary. What we aspire to is In these pages, we present an English version of the very first textbook on geometry, a book assembled by the Greek scholar Euclid in the 4th century B.C. It is the most remarkable textbook the world has ever seen: the Efforts have always been made to express the What distinguishes Euclid's text from today's, is that it is completely verbal. There is no algebra, no Now, we can angles at the base are also equal. Or, when two straight lines meet, the angles labeled 1 and 2 are equal. But what distinguished the Greeks is that they wanted to Plane geometry
In plane geometry we study figures that are flat, and their boundaries are called lines, or in the case of a circle, a single line. We say that a line, which may be straight or curved, is a length. We do not mean length as opposed to width; we mean any actual or potential boundary of a plane figure. A line is length with no width. That is the A line, too, may have its boundaries, or extremities, and we call them points. "Point" is the word we use, when we need it, to call attention to a specific place, such as where a line ends or where two lines meet. Points exist potentially. When we indicate a point, we may then say that it "exists." In plane geometry anything more than that is unnecessary. (See Is a line really composed of points?) We name a line by naming its extremities, its endpoints, with capital letters; thus we speak of the line AB. Or we could call it BA, it does not matter, except if we want to emphasize that it extends in one direction, we would call it AB; if in the other direction, BA. The space enclosed by the boundary -- the figure itself -- is called area. An angle is formed when two straight lines meet. The point -- the place -- where they meet is called the vertex. We name an angle with three letters -- "angle ABC" -- and we place the vertex in the center. When there is no doubt as to which point is the vertex, we may say "the angle at B," or simply "angle B." Magnitudes A magnitude is whatever has size: it could be larger or smaller. Length, area, and angle are the three kinds of magnitudes we study in plane geometry. We compare magnitudes of the same kind, and we try to decide how they are related. Two lengths, two areas, or two angles either will be equal to one another, or one of them will be larger. If we say, then, that these two triangles are equal -- -- we mean that they are equal areas. Because that is what kind of magnitude a triangle is. The space enclosed by each boundary will be exactly the same. A magnitude is not like a natural number, because a magnitude is continuous, while a natural number, which is a collection of indivisible units, is discrete. In algebra and arithmetic, the words Straight lines Each straight line that forms the boundary of a square, for example, will have two extremities; obviously. But we imagine that we could extend a straight line for as far as we please, and therefore we say that a straight line is potentially infinite. To be potentially infinite is in marked contrast to being actually infinite -- which would mean that the straight line has no endpoints at all. In plane geometry we are concerned only with what we can actually draw, and so we have no need even for the idea of actually infinite lines. (For they are never more than an idea; we cannot draw them, and they serve no practical purpose.) In what is called analytic geometry, however, they think they do need such lines. But in plane geometry, it is only finite lines -- the actual or potential boundaries of a figure -- that we ever require. Hence when we speak of a "straight line" in these pages, we mean what we can actually experience or draw: a line with two extremities. Those who think that even in plane geometry they need actually infinite lines, imagine that any finite line, such as a side of a square, is a segment -- a part -- of an actually infinite line. (Strange. No?) Hence they call a side of a square a "line segment." The idea in Euclidean geometry is quite the reverse, namely that we can extend any straight line for as far as we please. These quarrels about actual versus potential infinities arise only when straight lines are abstracted from the boundaries of figures. But in plane geometry it is the figures that concern us, and in the ultimate theorems straight lines appear only as their boundaries. Also, just because we can define something (such as an actually infinite line) does not guarantee that it exists. (We can define a unicorn. But does a unicorn exist?) As we shall see, the This completes the preliminary description, this is what plane geometry is about; we are now ready to study it as a logical science. Logic The method of logic is to know something or to prove it through reasoning. Logic provides one answer to the question, "How do I know?" One can reply, "I deduced it." (Latin Now, it is not possible to prove every statement. If we had to, there would be no end. Rather, to prove or explain anything is to do so in terms of something simpler, something that we already acknowledge and accept. Similarly, it is not possible to give a verbal definition of every word, because we must already understand the words in which a definition is framed. An example from arithmetic is the impossibility of defining Logic, then, which is the science of reasoning, is not 1) Definitions 2) Postulates 3) Axioms or Common Notions The statements found there will justify the statements we will make in proving the Theorems and Problems that follow. Before presenting the first principles, let us become familiar with the vocabulary of logic. Copyright © 2014 Lawrence Spector Questions or comments? E-mail: themathpage@nyc.rr.com |