In spite of the political decline of the Western Roman Empire, church building was vigorously pursued throughout the empire's territory during the so-called Early Christian period (i. e., a span equivalent to the early Byzantine period in the Eastern Empire). Moreover, the remains of ecclesiastical buildings from this period are to be regarded as prototypes on which later variations were based. It is important to clarify the notion of architectural composition in the early Christian buildings. As a first stage, the various building professions yield the suggestion that a certain architectural rationalism had by this time emerged from the general obscurity surrounding the building activity. The titles, or jobs, as listed in Table-1, sometimes appear vague in the context of our knowledge of contemporary society, if compared with their dictionary meanings. The most important of these are : οιχοδομζ, γεχντηζ, αρχιτεχτων, and μηχανιχοζ. In particular, μηχαωχοζ, which means literally' mechanical engineer', indicates a high-ranking profession resembling today's architect to a certain extent. However, detailed examination seems to show that the μηχανιχο, would almost certainly have been engaged in carrying out various scientific tasks in addition to the building activities to which G. Downey would restrict him. For such a person without regard to religious capabilities or aptitudes, architectural work was only one field among the many that could be referred to in general as the domain of applied science. He resembles a Leonardesque figure of the Italian Renaissance, and, as far as this was the case, his abilities surpassed those sketched out by Vitruvius. The results of my research suggest that the division of building labor and productivity must have been established during the Early Christian Peroid. From the fact that the μηχανιχοζ proceeded in his work against a background of non-occult, secular and 'scientific' knowledge, we may infer ecclesiastical architecture to have been characterized by a sort of newfound technical 'proto-rationalism'.