  In his Autobiographical Notes, Einstein mentioned that on his road to the
final theory of general relativity it was a major difficulty to accustom
himself to the idea that coordinates need not possess an immediate physical
meaning in terms of lengths and times. This appears strange: that coordinates
are conventional markers of events seems an obvious fact, already familiar from
pre-relativistic physics. In this paper we explore the background of Einsteins
difficulties, going from his 1905 paper on special relativity, through his 1907
and 1911 papers on the consequences of the equivalence principle, to the 1916
review paper on the general theory. As we shall argue, Einstein's problems were
intimately connected to his early methodology, in which clarity achieved by
concrete physical pictures played an essential role; and to the related fact
that on his route to the general theory he focused on special situations that
were easily accessible to physical intuition. The details of this background of
Einstein's early reasoning have not always been sufficiently appreciated in
modern commentaries. As we shall see, this has led to erroneous judgments about
the status and validity of some of the early relativistic derivations by
Einstein and others, in particular concerning the gravitational redshift.
