 Kant remarks that certain trends are compatible with progress, but cautions that no trajectory can be inferred with certainty from the facts (1784, 50).  His a priori argument begins with the premise that all animals have natural faculties.  If nature is not to be in vain, we must assume that the faculties of an animal can be developed.  Unlike other animals, the human being cannot develop all of its faculties in a lifetime.  If the faculties given to humans are not to be considered useless, then the only other possibility is that the human race as a whole, over time, will develop all the human faculties (1784, 42–44).  The progress from one era to another is measured by the development of human faculties during that time. 