A physical theory is a construction of thought which is founded on experience so as to constitute knowledge of the natural world. Propositions in physics are constituted of concepts which express the properties and processes of the physical world. For purposes of record and communication concepts are designated by the terms of a language, such as mathematical symbols, and philosophical discussion may be based on linguistic forms. In this essay, however, the element of discussion will be the concept as a (...) mode of thought through which are grasped the attributes of physical things and the relations exemplified in physical processes. The present philosophical analysis of a physical theory, quantum mechanics, is to be carried out by critical analysis of its basic concepts. In particular, the program is to show that in quantum mechanics the concept of physical objects exhibits the characters of discontinuity, complementarity, and relativity. (shrink)