It's Beck Ancall.
Rebecca Ancall was a hardworking, yet underappreciated 19th century English courtesan who coined such phrases as "I'm ready when you are" and "whatever you like". A mediocre chess player at best, to this day there is still disagreement on which opening she preferred. Never one to shy away from a challenge, she and a number of very enthusiastic volunteers established what later became known in some social circles as "the London System".
Yep, everyone is special. And when everyone is special, nobody is special.
To say that everyone trained at different times and had different resources is not to admit that they were all equal. If, despite all the resources at Magnus' beck and call, he is only equal to someone from 50 years ago, that would mean he's pretty weak.
Just the opposite has happened. Magnus is clearly the best player in history, and we know that in part by analyzing the games and in part by looking at the ratings, which have been *deflated* over time.
The reasons for Magnus' domination can be discussed, but to say that Fischer or anyone else from the 1970s is equal to Magnus is to denigrate his amazing accomplishments. What Fischer did in his own time may never be duplicated, but the same is also true for Karpov, Kasparov, and Carlsen.
We can still compare the relative quality of their play though.
Beckon call*
I agree that saying that the players are equally strong would be to imply that later players are weaker than they are; it's fairly clear that each in their peaks Magnus would beat Kasparov who would beat Fischer, and this is due to their access to differing resources. The question still remains, given their differences in resources, if we account for that who is the best, and how do we go about that