"[W]hen we have ideas and pictures that are extremely useful, they acquire elements of reality in and of themselves. But, philosophically, it is instructive to look at the degree to which such objects are purely instrumental—merely useful tools—and the extent to which physicists seriously suppose they embody an essence of reality... it is possible to view the renormalization group as merely an instrument or a computational device. On the other hand, at one extreme, one might say: ‘‘Well, the partition function itself is really just a combinatorial device.’’ But most practitioners tend to think of it (and especially its logarithm, the free energy) as rather more basic!"