sn y™to˜er —nd xovem˜er IWHUD just over two ye—rs —fter the ™ompletion of his spe™i—l theory of rel—tivityD iinstein m—de the ˜re—kthrough th—t set him on the p—th to the gener—l theory of rel—tivityF ‡hile prep—ring — review —rti™le on his new spe™i—l theory of rel—tivityD he ˜e™—me ™onvin™ed th—t the key to the extension of the prin™iple of rel—tivity to —™™eler—ted motion l—y in the rem—rk—˜le —nd unexpl—ined empiri™—l ™oin™iden™e of the equ—lity of inerti—l —nd gr—vit—tion—l m—ssesF „o interpret (...) —nd exploit this ™oin™iden™eD he introdu™ed — new —nd powerful physi™—l prin™ipleD soon to ˜e ™—lled the ’prin™iple of equiv—len™e4 upon whi™h his se—r™h for — gener—l theory of reE l—tivity would ˜e ˜—sedF woreoverD with the ™ompletion of the theory —nd throughout the rem—inder of his lifeD iinstein insisted on the fund—ment—l import—n™e of the prin™iple to his gener—l theory of rel—tivityF iinstein9s insisten™e on this point h—s ™re—ted — puzzle for philosophers —nd histori—ns of s™ien™eF st h—s ˜een —rgued vigorously th—t the prin™iple in its tr—dition—l formul—tion does not hold in th™ gener—l theory of rel—tivityD gonsiderD for ex—mpleD — tr—dition—l formul—tion su™h —s €—uli9s in his IWPI Encyklopadie —rti™leF por €—uli the prin™iple —sserts th—t one ™—n —lw—ys tr—nsform —w—y —n —r˜itr—ry gr—vit—tion—l eld in —n innitely sm—ll region of sp—™eEtimeD ˜y tr—nsforming to —n —ppropri—te ™oordin—te system @€—uli IWPID pF IRSAF sn responseD su™h eminent rel—tivists —s ƒynge @IWTHD pF ixAD —nd even iddington ˜efore him @IWPRD ppF QW{RIAD h—ve o˜je™ted th—t — ™oordin—te tr—nsform—tion or ™h—nge of st—te of motion of the o˜server ™—n h—ve no ee™t on the presen™e or —˜sen™e of — gr—vit—tion—l eldF „he presen™e of — ’true4 gr—vit—tion—l eld is determined ˜y —n inv—ri—nt ™riterionD the ™urv—ture of the metri™F „he gr—vit—tionEfree ™—se of spe™i—l rel—tivity is just the ™—se in whi™h this ™urv—ture v—nishesD where—s the true gr—vit—tion—l elds of gener—l rel—tivity —re distinguished ˜y the nonv—nishing of this ™urv—tureF „his o˜je™tion h—s immedi—te r—mi™—tions for the ’iinstein elev—tor4 thought experimentD whi™h is ™ommonly used in the formul—tion of the prinE ™iple of equiv—len™eF sn this thought experimentD — sm—ll ™h—m˜ers su™h —s.... (shrink)