This article is intended as an addition to the book [10], since, in the first edition, I was double minded whether to introduce the Dirac theory for young students. Now I am quite sure that it should be introduced, and for several reasons. First, the Cl3 formulation of the Dirac theory is simple and the derivation of the Dirac’s formula is straightforward. Second, it is relatively easy to show that gamma matrices are not the only possibility in linearizing the Klein-Gordon equation (we even do not need it in Cl3). Finally, the fact that it is possible to use the same mathematical (3D) formalism for classical mechanics, the special (and general) theory of relativity (without Minkowski space), electromagnetism, and both non-relativistic and relativistic quantum mechanics (without the imaginary unit) is remarkable. Not to mention the geometric clarity and possibilities of unifications, as well as generalizations. Moreover, all this without coordinates, matrices, tensors… In addition, we should appreciate the new concept of oriented numbers and simple fact that Cl3 contains complex, hypercomplex, and dual numbers, quaternions, spinors, etc. Geometric algebra of 3D Euclidean vector space (Cl3) is truly rich in structure and the question remains as to how physics would have developed had the ideas of Grassmann and Clifford been accepted in the late nineteenth and early twentieth centuries.