The processes of lubrication are systematically analyzed and explained with the aid of recent experimental discoveries. Effective lubrication often involves surface chemical reactions which are dependents on contact geometry, load, speed, and environmental influences. Materials properties such as hardness, elasticity, and others also affect the wear outcome. Fluid mechanics under the influence of interfacial pressures and temperatures controls a significant portion of the wearing processes. A conceptual model is proposed to link all these factors in a 'lines of defense' framework and to discuss how wear can be predicted based on a probabilistic model. The model proposes a series of criteria of various combinations of chemically controlled and materials properties controlled situations.