Ideas developed to describe the simulated glass transition in simple models, like hard spheres, are applied to real materials in which temperature, rather than density, is the important variable. The free energy of a liquid is expressed in terms of the free energy of all the glasses that it samples;The assumption that all glasses of the same substance have the same heat capacity allows an estimate of the number of glasses that it can form and the distribution of their enthalpies, from heat capacity measurements on the liquid and an experimental glass. The results suggest: a thermodynamic glass transition underlying the experimental kinetic transition, The same ideas explain the relation between thermodynamic and kinetic measures of fragility in liquids, and they show that the fragility of a liquid is directly related to the total number of glasses that the material can form.