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A conception of the Correspondence Principle which Bohr deployed implicitly in developing a new theory of atomic constitution in 1913, is made explicit through an extensive examination of his classic paper of that year. Arguments are considered which purport to show that the application of the principle must be restricted to few isolated cases. These arguments are either defused or rejected. In particular an extensive review of issues concerning the interpretation of Quantum Mechanics is made to counter the claims that an insurmountable conceptual gap exits between the tenets of this theory and those of Classical Mechanics which makes it logically impossible for the latter to be regarded as the 'limiting case' of the former. In the light of a particular interpretation adopted and defended, a proposal is made that suggests that the Hamilton-Jacobi formulation of Classical Mechanics, as well as Maxwell's electromagnetic theory, can be viewed as 'limiting cases' of Quantum Mechanics. Having established a case for the global validity of the requirement imposed on physics by the Correspondence Principle, it is then argued that this requirement is indispensable if a particular brand of realism is adopted for the interpretation of theories in physics. Taking on board the assumption that an ultimate theory exists which mirrors the underlying physical constitution of the world, it is subsequently argued that the intertheory order established by the global imposition of the principle in physics, can be used to solve the problem of rational theory choice for this brand of realism.

This thesis examines the concepts of identity and individuality via scientifically-informed philosophical analysis. It has two parts. The first part deals with metaphysical claims that turn out to be, in effect, very general empirical claims extracted from our (supposed, but rationally-accredited) knowledge of the world. I here compare two approaches: a) the Leibniz-Quine view of identity as a derivative relation, and, relatedly, of individuality as dependent on the qualities of things; and b) the view that identity is a non-analysable primitive, and, relatedly, that the individuality of things is not reducible to anything else. The former position, based on the Principle of the Identity of the Indiscernibles as a criterion of individuation, might appear prima facie more plausible. However, I argue that it runs into difficulties both at the level of a priori analysis and in terms of 'fit' with the evidence described by our best science. It is, in fact, not even as compelling from the empiricist point of view as is commonly believed. I therefore argue that the position that identity and individuality are primitive may be preferred. In the second part of the thesis - under the assumption that the proper role of metaphysics is to characterise the best solutions to issues that are left open by current science - I deal with the question regarding whether and how the ultimate constituents of reality can actually be conceived of as primitive individuals. I argue in favour of an ontology of tropes, develop the view in detail and defend it against various criticisms. The fundamental tropes that constitute the basic 'building blocks' of reality are identified with the state-independent properties of elementary particles. The way in which these constitute complex particulars is described, and certain peculiarities having to do with quantum statistics are accounted for. I conclude by suggesting possible avenues for further research.

The development of quantum protocols from conception to experimental realizations is one of the main sources of the stimulating exchange between fundamental and experimental research characteristic to quantum information processing. In this thesis we contribute to the development of two recent quantum protocols, Universal Blind Quantum Computation (UBQC) and Quantum Digital Signatures (QDS). UBQC allows a client to delegate a quantum computation to a more powerful quantum server while keeping the input and computation private. We analyse the resilience of the privacy of UBQC under imperfections. Then, we introduce approximate blindness quantifying any compromise to privacy, and propose a protocol which enables arbitrary levels of security despite imperfections. Subsequently, we investigate the adaptability of UBQC to alternative implementations with practical advantages. QDS allow a party to send a message to other parties which cannot be forged, modified or repudiated. We analyse the security properties of a first proof-of-principle experiment of QDS, implemented in an optical system. We estimate the security failure probabilities of our system as a function of protocol parameters, under all but the most general types of attacks. Additionally, we develop new techniques for analysing transformations between symmetric sets of states, utilized not only in the security proofs of QDS but in other applications as well.