Quantum mechanics is more than a physical theory of nature. It is a quantitative philosophy that provides us with a set of general， overarclung principles that describe the innermost workings of our world at its most fundamental level. Due to its central importance， all physical and philosophical theories have to be consistent with quantum mechanics. There are two aspects of quantum mechanics： the "machinery" and the "spook." The machinery， epitomized by the Schrodinger equation and its various methods of solution， allows us to propagate the quantum state of a system deterministically forward in time. This aspect of quantum mechanics is not particularly "quantum；" we find it in similar form in all classical field theories. The "spook" are all those aspects of quantum mechanics that do not have a classical analogue， not even in principle. This part of quantum mechanics is connected with the theory of measurement and its implications. Since quantum mechanics is the most basic theory of nature， no wonder that quantum mechanics is capable of confronting basic philosophical questions head-on， prime among them the nature of reality. 　　For the longest time， the philosophical implications of quantum mechanics were far from main-stream physics. Only very few physicists studied them because it was felt that this is idle speculation， alien to a "hard，" quantitative science such as physics. In particular， physicists thought that there are no consequences or applications. This point of view was fueled by the immense success of the quantum mechanical machinery， which， like a steam roller， flattened and solved any "practical" problem it was applied to. In such an atmosphere of success， questions probing the foundations of the theory appeared irrelevant and futile. 　　"Shut up and calculate" was the standard response of teachers and research advisers to students of quantum mechanics raising questions concerning the meaning and interpretation of quantum mechanics.