Knowledge within a firm comes in two forms. Codified knowledge, blueprints, manuals, written procedures, can be articulated in symbols and therefore copied, transmitted, and stored at low cost. Tacit knowledge resides in the practiced judgment of individuals and the coordinated habits of teams: the machinist who hears when a tool is about to fail, the surgical team whose members anticipate one another without speaking. Such knowledge is difficult to put into words precisely because those who possess it often cannot fully specify what they do. This distinction has a paradoxical strategic consequence. A capability built largely on codified knowledge is easy for a firm to transfer to a new plant or a new market, but for the same reason it is easy for rivals to imitate once the codification leaks or is reverse-engineered. A capability rooted in tacit knowledge is the mirror image: hard to move even within the firm, since a new plant cannot simply read the recipe and reproduce the result, yet correspondingly hard for competitors to copy, because there is no recipe to steal. The very feature that makes tacit knowledge a barrier to internal replication makes it a moat against external imitation. Managers sometimes try to capture the best of both by codifying tacit knowledge, writing down what the experts know. The effort can succeed at the margin, but the passage's logic warns of a cost: to the extent codification succeeds, it converts a protected capability into a copyable one. A firm that fully documents the source of its advantage may find it has also handed rivals the means to erase it.