This work ends a trilogy devoted to a journey into the foundations of special relativity. The first paper debated the meaning of the constancy of the two-way speed of light and its close relation to the conceptualization of time. The second one addressed the question of the possible constancy of the one-way speed of light and the trivial--but, unfortunately, even now somewhat controversial--question of the compatibility between the assumption of a special system of reference and Einstein's special relativity. The present study deals with the principle of relativity. Its historical evolution is reviewed and a "weak" formulation is defended. It is emphasized that many assertions usually associated with special relativity, such as the "relativity of time dilation" and "relativity of space contraction" are indeed "philosophical statements", as it has been established already by several authors in the past. Nonetheless, most teachers and scientists still believe nowadays they are implied by the theory and by the group property of the Lorentz transformation. This is by no means so, as it is reviewed and elucidated with the simple example on space contraction. It is argued that the lack of knowledge of the true value of the one-way speed of light in empty space leaves the theory "undetermined". Einstein's special relativity corresponds to a simple and very elegant solution to this problem, allowing the study of relative motion without any concern with the study of absolute motion, which is considered to be superfluous. However, its standard interpretation is minimalist and even misleading. A large number of researchers have discussed this question, mostly within the "conventionality" of simultaneity thesis. The typical formulation of this thesis provides some new physical insight and points out the problem, but does not solve it. In contrast, it often leads to a labyrinth of difficult language which is herein clarified. (Contains 2 figures and 2 footnotes.)