This paper examines the idea, commonly presupposed but seldom explicitly stated in discussions of certain philosophical problems, that the objects and phenomena of the world are structured in a hierarchy of "levels", from the bottom level of microparticles to the levels of cells and biological organisms and then to the levels of creatures with mentality and social groups of such creatures. Parallel to this "layered model" of the natural world is an ordering of the sciences, with physics as our "basic" (...) science and the "special sciences" forming a ladder-like hierarchy, from chemistry to biology to psychology and the social sciences, all grounded in basic physics. Focusing on two formulations of this model, the emergentist model of C. Lloyd Morgan and the reductionist model due to Paul Oppenheim and Hilary Putnam, the paper discusses such questions as these:What makes a given level of objects "higher" or "lower" than another? What makes a given property "higher" or "lower" than another property. Are objects and properties always comparable with respect to "higher" and "lower"? Can an overarching hierarchy of entities and properties be developed that comprehends the entire natural order? The issues turn out to be quite complex, and no definitive general conclusions are reached. (shrink)