 In fact, Poincaré’s model of explanation, founded on a minimum of well-confirmed hypotheses erected into principles and from which any meaningful propositions can be deduced, is called into question by Maxwell’s approach. In view of the crisis in physics that was brewing toward the end of the 19th century, Poincaré’s general position will not work without posing problems with respect to concrete principles. We will briefly mention the most interesting case, that of the application of the principle of relativity to electrodynamics, which is one of the most discussed subjects by historians (e.g., Goldberg 1967; Paty 1993; Miller 1996a; Zahar 2001; Darrigol 2004; Rouché 2008; Walter 2011, DiSalle 2014)—not least because Poincaré and Einstein did not cite each other on relativity despite the fact that Einstein read Poincaré’s Science and Hypothesis before 1905 and Poincaré wrote Einstein a recommendation letter sometime after their only meeting at the first Solvay Congress in 1911 (Walter 2007: chap. 59.3). What is uncontroversial is that Poincaré discovered salient points of the special theory of relativity, such as an operational definition of clock-synchronization to the first order in \(v/c\), and a relativistic formula of the composition of speed, the determination of the structure of the Lorentz-group.