on the other hand is supposed to be an UV completion of field theory (or at least of a class of field theories hopefully including the standard model). The theory is not supposed to have any free parameters and is defined purely in terms of a fundamental dimensional constant - the string scale $l_{string}\equiv\sqrt{2\pi\alpha'}=1/M_{string}$. If string theory were four dimensional, $M_{string}$ would essentially be the same as $M_{P}$. However in all tractable string theoretic constructions, there is an internal six-dimensional space with a volume which is typically large compared to $l_{string}^{6}$, i.e. ${\rm Vol}={\cal V}l_{string}^{6}$ often with ${\cal V}\gg1$. In this case there is a significant difference between the two scales and $M_{S}\simeq M_{P}/\sqrt{{\cal V}}\ll M_{P}$. There is also an additional scale, the Kaluza-Klein scale $M_{KK}=M_{string}/{\cal V}^{1/6}=M_{P}/{\cal V}^{2/3}$. For large ${\cal V}$ we thus have a hierarchy of scales $M_{KK}\ll M_{string}\ll M_{P}.$ Four dimensional field theory is strictly valid only below $M_{KK}$. Above this scale the theory is essentially ten dimensional but remains a field theory. However above the scale $M_{string}$, the theory *cannot* be described by point like field theoretic degrees of freedom. The field theoretic description necessarily breaks down.

Consider first the case of strings propagating in a 