to have been the most fruitful cause of international differences; (7) in the fact, which is receiving ever-greater recognition by modern economists and statesmen, that the innate quality of the object for good or harm must be considered in a true definition of economic wealth.[110] PRODUCTION Plato seems to have had little positive interest in the problems of production. He was too much engrossed with suggesting means for limiting excessive acquisition. He was, however, quite apt in his use of illustrations from industrial life.[111] He was also apparently the first to give a real classification of trades,[112] as follows: furnishers of raw materials (πρωτογενὲς εἶδος), makers of tools (ὅργανα), makers of vessels for conserving products (ἀγγεῖα), makers of vehicles (ὅχημα), manufacturers of clothing and means of defense (προβλήματα), workers in fine arts (παίγνιον), producers of food (θρέμμα)—a fairly inclusive catalogue for that age; if commerce and the learned professions were included. But some of the classes overlap, since they follow no necessary principle of division. He divided productive arts into co-operative (συναιτίους), which provide tools for manufacture, and principal (ἀιτίας), which produce the objects themselves.[113] They were further divided into productive arts (ποιητικαί), which bring something new into existence, and acquisitive (κτητικαί), which merely gain what already exists. In the latter class, he placed all commerce, science, and hunting.[114] Plato would thus appear to exclude commerce and the learned professions from the true sphere of production. This, however, is only apparent, in so far as legitimate exchange is concerned. He clearly understood that the merchant and retailer save the time of the other workers,[115] and that they perform a real service to the community, in that they make necessary exchange convenient and possible.[116] He thus recognized them as producers of a time and place value, and he cannot be accused of the physiocratic error, which denied productivity to all workers except those who produce directly from natural resources.[117] His distinction of productive and acquisitive arts can, furthermore, hardly be interpreted as intending to limit production to the material merely, though learning is relegated to the acquisitive class. Such an interpretation would be out of harmony with the whole trend of his thought.[118] His further classification of productive agencies as creative (ἕνεκα τοῦ ποιεῖν τι) or preventive (τοῦ μὴ πάσχειν)[119] substantiates this, for many of the preventive agencies are intellectual and scientific.