Quantisation as a method of generation: The nature and prospects of theory changes through quantisation.

Quantisation is a highly successful heuristic tool for constructing a quantum theory out of a classical theory --- even though it is typically accepted that the classical is derivative on the quantum. With this paper, I aim to show that there are overlooked philosophical issues concerning theory generation, and theory change, that appear when considering the quantisation of classical theories. More concretely, I will reveal three major challenges to quantisation qua method of generation and discuss how they can be met. To begin with, given that quantisation goes the wrong way (at least on any orthodox account on the relationship between the classical and the quantum), it must involve ambiguities. A central task in understanding quantisation better as a method of generation is thus to investigate the scope of these ambiguities as well as possible remedies to them. This will be the topic of the first half of the paper. Ambiguities are, however, just one aspect --- albeit a central one --- of quantisation qua method of generation. How to think of quantisation as method of generation overall? In the second part of the paper, I offer a conceptualisation of quantisation as a form of prescription for translating between two theoretical frameworks --- namely a classical and a quantum one. Making use of the characterisation of quantisation as prescriptional theory change, we make a first stab at assessing quantisation with respect to two other central issues of a generative strategy (over and above the ambiguity issue): that is how deep the structure it generates is (deep structure issue), and how it is justified (justification issue). Independently of my hope to offer the reader various specific insights on quantisation as a method of generation, this paper has already achieved its goal if it manages to reveal and map out the rich but unappreciated philosophical problems associated with quantisation qua method of generation.

Miracles persist: a reply to Sus.

In a recent article in this journal, Sus purports to account for what have been identified as the 'two miracles' of general relativity-that (1) the local symmetries of all dynamical equations for matter fields coincide, and (2) the symmetries of the dynamical equations governing matter fields coincide locally with the symmetries of the metric field-by application of the familiar result that every symmetry of the action is also a symmetry of the resulting equations of motion. In this reply, we argue that, while otherwise exemplary in its clarity, Sus' paper fails in this regard, for it rests upon a illegitimate application of the aforementioned result. Thus, we conclude, pace Sus, that these two miracles persist in general relativity.

The strong arm of the law: a unified account of necessary and contingent laws of nature.

A common feature of all standard theories of the laws of nature is that they are 'absolutist': They take laws to be either all metaphysically necessary or all contingent. Science, however, gives us reason to think that there are laws of both kinds, suggesting that standard theories should make way for 'non-absolutist' alternatives: theories which accommodate laws of both modal statuses. In this paper, we set out three explanatory challenges for any candidate non-absolutist theory, and discuss the prospects of the two extant candidates in light of these challenges. We then develop our own non-absolutist theory, the essentialist DTA account, which combines the nomic-necessitation or DTA account with an essentialist approach to metaphysical modality in order to meet the three explanatory challenges. Finally, we argue that the distinction between kinematical and dynamical laws found in physical theories supports both non-absolutism in general and our proposed essentialist DTA view in particular.

The modal status of the laws of nature. Tahko's hybrid view and the kinematical/dynamical distinction.

In a recent paper, Tuomas Tahko has argued for a hybrid view of the laws of nature, according to which some physical laws are metaphysically necessary, while others are metaphysically contingent. In this paper, we show that his criterion for distinguishing between these two kinds of laws - which crucially relies on the essences of natural kinds - is on its own unsatisfactory. We then propose an alternative way of drawing the metaphysically necessary/contingent distinction for laws of physics based on the central kinematical/dynamical distinction used in physical theorising, and argue that the criterion can be used to amend Tahko's own account, but also that it can be combined with different metaphysical views about the source of necessity.

Aggregate geometry in amyloid fibril nucleation.

We present and study a minimal structure-based model for the self-assembly of peptides into ordered ß-sheet-rich fibrils. The peptides are represented by unit-length sticks on a cubic lattice and interact by hydrogen bonding and hydrophobicity forces. Using Monte Carlo simulations with >10(5) peptides, we show that fibril formation occurs with sigmoidal kinetics in the model. To determine the mechanism of fibril nucleation, we compute the joint distribution in length and width of the aggregates at equilibrium, using an efficient cluster move and flat-histogram techniques. This analysis, based on simulations with 256 peptides in which aggregates form and dissolve reversibly, shows that the main free-energy barriers that a nascent fibril has to overcome are associated with changes in width.