Regularity can be taken as a sign of causation but it is also true that once a cause occurs, its usual effect might still not happen. There are preventions and interferences that can stop a cause from having its typical effect and this explains why prediction is fallible. A positive way to look at the same issue, however, is that intervention and manipulation are possible in cases where it is useful to stop a cause from doing its work or to change its effect in some way. Subtractive and additive interference can be distinguished, the latter especially posing a challenge to regularity accounts of causation.

Hume said that necessity was part of the popular concept of cause but not legitimately so. Necessity could be found in no experience of causation so should be expelled from the concept. To this extent, Hume was right but it leaves us with a problem of inductive scepticism. Nevertheless, many of his critics overreacted in defending the necessity of causation. Additive interference shows that there is no causal necessity. Both Hume’s thesis and its antithesis seem flawed; but there are prospects of a synthesis. The idea of tendency can give us an intermediate, third modality between necessity and pure contingency, and this seems to be the correct modality of causation.

While the world growth in PVC consumption continues (currently ~35 million tonnes per annum), its poor thermal stability, remains an issue due to restrictions on the use of stabilizers containing heavy metals such as lead. Monitoring degradation in PVC is important, and this work is concerned with the detection of PVC degradation using UV and Raman spectroscopies. The bands which appear in the Raman spectra of PVC samples after thermal degradation at around 1100 and 1500 cm-1, are the result of a resonance Raman effect. Optimization of the PVC degradation assessment process using laser-Raman is crucial to minimize the effects of additive interference and the fluorescent background. The most important factor is the wavelength of the exciting laser source, as it should fall within the absorption range for polyene sequences in degraded PVC. Laser-Raman spectroscopy has been shown to be an appropriate method for the quantitative determination of the polyene length distribution in lightly degraded PVC samples. The application of this method for various degraded PVC samples is reported. Ageing of compounds containing various heat stabilizers were considered, in order to assess their effectiveness. Additives studied included calcium zinc stabilizers and two commercial grades of hydrotalcite (Sorbacid 911 and Alcamizer P93).