The Spanish Dominican friar Juan de Ortega wrote the first book on commercial arithmetic to be published in French in 1515. The five centuries that separate us from his writing lead us to frequent misinterpretations due to the old fashioned style, words and contents. For this reason, it was worth reading by high school students under teacher’s guidance. They would discover old methods of multiplying, like the gelosia or others, hardly understandable because they really look like our contemporary techniques but resist interpretation. They would try to make sense of problems of sharing money according to the will of a dying father, to find the cost of a horse whose purchase is made by several cranky merchants who firmly refuse to reveal their share, and so on. Studying the text involves wondering about the meaning of words and explaining algebraically various methods of solving equations without algebra. So the pupils had to leave their highly polished contemporary methods to venture into interpretation. Fortunately, it is a matter of mathematics and this language remains understandable over the centuries. This chapter is an invitation to experience how the light can be born from obscurity.