This book, like the others in the Series, is written in simple English – the language most widely used in science and technology. It takes the next big step beyond “Number and symbols” (the subject of Book 1), starting from our first ideas about the measurement of distance and the relationships among objects in space. It goes back to the work of the philosophers and astronomers of two thousand years ago; and it extends to that of Einstein, whose work laid the foundations for our present-day ideas about the nature of space itself. This is only a small book; and it doesn’t follow the historical route, starting from geometry the way Euclid did it (as we learnt it in our schooldays); but it aims to give an easier and quicker way of getting to the higher levels needed in Physics and related sciences.