 Einstein’s general theory of relativity (GTR) was also constructed using a symmetry principle at its heart: the principle of general covariance. Much ink has been spilled over the significance and role of general covariance in GTR, including by Einstein  himself.[10]  For a long time he viewed the principle of general covariance as an extension of the principle of relativity found in both classical mechanics and STR, and this is a view that continues to provoke vigorous debate. Norton (2003) discusses the “Kretschmann objection” to the physical significance of general covariance. On invariance versus covariance, see Anderson (1967), Brown and Brading (2002), and Martin (2003, Section 2.2). What is clear is that the mere requirement that a theory be generally covariant represents no restriction on the form of the theory; further stipulations must be added, such as the requirement that there be no “absolute objects” (this itself being a problematic notion). Once some such further requirements are added, however, the principle of general covariance becomes a powerful tool. For a recent review and analysis of this debate, see Pitts (2006).