Mathematics is the science with the least investment and the highest results - it does not require more than a pencil and paper, sometimes even less, because the only (re)source of mathematics is the truth. Mathematics shows us that simple things are the valuable ones. It teaches us that maximum precision is not necessarily obtained with complex devices: a simple argument can be the most exact instrument if it is in accordance with the frequency of truth. That is why I believe that it can be defined at least in a satisfactory way, as follows: Mathematics is the science of expressing the truth in all the categories used: concrete, abstract, ideas, methods, objectives, hypotheses, results, applications, etc. Mathematics is an instrument of knowing the truth.