A rumination on managerial judgment

My opening assumption is that managing is not usefully defined as an abstraction, a computer-like decision-making activity; rather it should be defined by the nature of the managerial task - being shaped by the nature of the thing being managed (the firm) with needs only its managers can address. But what is "the firm" that thereby defines "managing"? Management and organization theorists take "the firm" pretty much for granted - IBM exists, just as Facebook and Alitalia, exist for the moment. We have ample evidence that firms exist - just ask people which they work for. Likewise the basic nature of "business management" is generally assumed - firm-related decision-making, idealized as "rational" but certainly presumed key to the firm's practices. Assuming firms exist, we organization theorists discuss making rational decisions about their structure, process improvement, hiring, payment systems, marketing, alliances, product mix, IT needs, culture, relative performance, and so on. The firm's existence seems as obvious as the goods and services it produces - only the selection and direction of the firm's various investments and activities seems problematic, non-rational to the extent managers fail to see the firm's truly rational nature.