Mach's doctrine of the economy of thought, or Denkökonomie, has long been recognized as one of his most important philosophical achievements. He claimed to have held it as early as 1861, when he began his Privat-Dozent lectures, and expressed it first in the Conservation of Energy.¹ Subsequent treatments occurred in his lecture "On the Economical Function of Physical Theory" (1882), the Mechanics, and chapters in his later writings, the Theory of Heat and Knowledge and Error.